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In several places where I get pronunciations I've come across 45 being pronounced something like fümundvierzig or possibly fünwundvierzig, not the f sound that I would expect. Is this Hochdeutsch and I've just missed this, or is this a regional dialect? Danke!

Wiktionary is available in more than 150 different languages, and in each language it contains descriptions of the most frequent used words in this language. German Wiktionary more than 750,000 entries, and for each word you can find there the correct pronunciation.

The page about the word fünfundvierzig says, that there are two proper pronunciations for this word:

• /ˌfʏnf(ʔ)ʊntˈfɪʁtsɪç/ (standard)


• /ˌfʏmm̩ˈfɪʁtsɪç/ (colloquial)

German Wikipedia has a very good list of all IPA symbols. I think it is much better than the English page. But I will explain what you need here:

The easy consonants:

• 
  f = f in "fill"


• 
  m = m in "milk"


• 
  n = n in "tin"


• 
  t = t in "tin"


• 
  s = s in "sea"

The other consonants:

• 
  ʁ = r in "red".
  
  This one of the three ways to pronounce the letter R in German. The other two are [ʀ] and [r] which both doesn't exist in standard English. In German you always can replace each of them with one of the others. When r is at the end of a syllable (which is the case here), it often is replaced by a schwa sound (symbol: ɐ), like at the end of German jeder which sounds more like the vowel a than like a consonant. (You find ɐ as a writen u in English "nut", but ɐ is always unstressed in German)


• 
  ç = ch in German "ich"
  
  Note that ç is not c! (They are symbols for different sounds, but c is neither used in English nor in standard German)
  The sound ç doesn't exist in standard English. You need to learn it when you want to learn German.


• 
  ʔ
  This consonant is a glotal stop. This consonant has no written representation in German or English (but in some other languages like Arabic, Hebrew or Cyrillic), so many people speaking Germanic languages are not aware that it even exists, but they use it when they speak. In English it is the sound in the middle of "uh-oh" (here represented by a dash) and in German you find it between e and a in "beachten".
  
  The brackets around the consonant mean, that it is optional. So you can speak the standard version of this word with or without a glotal stop.


• 
  m̩
  Do you see the little vertical line below the letter m? This tiny symbol means, that the sound m here is no longer a consonant. At it's position within the word it behaves like a vowel, and it even is the core of a syllable.
  
  Here, in the colloquial version of this word, m̩ is a syllable on its own.

Now the vowels:

• 
  ɪ = i in "ink"


• 
  ʊ = oo in "book"


• 
  ʏ = ü in German "Nüsse"
  
  This sound does not exist in English. It is used when the letter ü has a short pronunciation. You need to learn it when you learn German.

The other symbols:

• 
  ( ) = optional
  
  (see ʔ above)


• 
  ˈ (ˈxxx) = main stress
  
  The syllable following this symbol is the most stressed syllable of the word


• 
  ˌ (ˌxxx) = secondary stress
  The syllable following this symbol is stressed, but it is not the most stressed syllable of the word.

I think the standard pronunciation is clear now. It is (spaces and underline separate syllables, the underline should express, that fiatsich is spoken in one part:

fünf und fia_tsich

The colloquial version:

füm m fia_tsich

The colloquial version is neither Standard German nor any dialect. It is a level between both of them. Professional speakers don't use this level when they do their job, so it is not part of the standard. But it is so wide spread across the German speaking area that you can't say it was part of a local or regional dialect.

Pronunciation is always a bit different from what spelling would suggest. One can call that sloppy pronuncation, but actually this happens, to some extent, also in totally well-educated pronuncation as e.g. in broadcasting or in theatre.

Examples for sloppy (but very common) pronuncation:

Guten Abend --> Nahmt! (Very sloppy)

Grüß Gott --> Sgott! (Happens only in the South, as only people in the South say "Grüßgott" instead of "Guten Tag".)

Entschuldigung! --> Schullijung!

Können Sie mal halten? --> Könnse mal haltn?

Not sloppy but standard German pronuncation

Lehrer --> Leera [roughly so; actually we would need to use phonetic notation to show the fine nuances; the a is not a full a, it is actually a Schwa vowel]

gehen --> gehn (it is completely normal to leave out the second e)

Schwimmflügel --> schwümmflügl

Your example

fünfundvierzig --> fünn-nfirtsich / fünn-wunfirtsich / fümm-wn-fyrtsych and so on.

This is not dialect. Dialect would e.g. be

fünfundvierzig --> fajwa-fyttsk (Swabian, slightly South of Ulm)

Note that very few language have a full congruence of spelling and pronuncation. Take English! A total mess, actually. Even French is more consistent in its spelling-pronuncation relation than English.

A relatively close relation between spelling and pronuncation is to be found e.g. in Serbian (one of the South-Slavonic language). That's because orthographic rules in this language were designed relatively lately in the 19th century, and purposefully based on how people pronounced things in real life. This was done, of course, at the cost of losing sight of etymology. The capital city of Serbia, Belgrade, meaning "White town", as "bjal" is "white" in Slavonic languages, thus has become "Beograd" in Serbian, where the "o" is foreign to etymology, but people in that region tend to pronounce the "l" quite like what you would actually think is an "o".

Another example for a language with close link between pronuncation and spelling is Turkish (more exactly: the Turkish used in Turkey, as there are other forms of Turkish in neighbouring countries). This is because the spelling rules were created just in the early 1920s, when the country moved from Arabic scripture (as in Ottoman Turkish) to modern Latin scripture including many Umlaut-vowels, especially ü and ö:

Müdürmüsünüz? (Are you the boss?)

You pronounce that exactly like that.

Words in isolation are pronounced differently from words in context. How big the difference is depends on the speed of articulation and the effort the speaker is making. What kind of modifications are allowed is dictated by the language and may vary from region to region.

The process you noticed is a voiceless consonant [f] becoming voiced [v]. This happens in other contexts as well.

fünfundvierzig [v]

Da muß ich noch drüber nachdenken. [z]

Das hat er doch gesagt. [d]

(Unless your pronunciation is already excellent, I would not recommend emulating this. You should first get the most explicit forms correct.)

As has been mentioned in other answers, there are other modifications.

assimilation of place of articulation: fünf [m], haben [m]

assimilation of voicelessness: hat sich [s] (instead of [z])

assimilation of nasality: eben [mm] (via [bm])

t-elision: Is nich dein Ernst!

Wiktionary's /fʏmm̩ˈfɪʁtsɪç/ for fünfundvierzig is a result of several of these modifications being combined. This is part of the standard language, just as the following is:

Hat er [ɐ] das/es [s] denn gewußt?

As in English, certain words have established weak forms. The first three words of the above sentence are one syllable!