Maybe a stupid question but i have just bought a complete build in a small, if i buy a medium frame will all the components fit, in terms of cables being long enough etc. thanks

I'd worry most about the steerer on the forks. Are there any spacers above or below the stem on your small build? What's the bike?

Almost certainly will need new cables, steerer tube as above.

Are we assuming that the medium is the same frame type as the small? If not, then seatpost & front mech clamp size might also differ.

As long as you have at least 15mm of spacers under your headset you should be fine, very few brands change by more than that.

Hoses and cables will almost certainly be an issue though.

depends on the brand, most use the same sizes through the range, others specify smaller seatposts etc on smaller sizes.

its a small boardman pro but want to move to an inbred

You're in luck with the steerer. The Inbred actually has a 15mm shorter head tube than the Boardman (same on all sizes).

does the boardman have gear cables in bits? the on one will need full length anyhow.. is the seatpost the same diameter?

Might have 'hose' issues, plus the On Ones run full outers, so you'll need them anyway.