I know some guys here use cycle computers for a speedo - what ones would you recommend? I imagine I'm looking for things like reasonably large display, waterproof, wheel measurement accuracy and so on.

halfords £10 i got on mine its spot on up to 130mph but no back light

You can get a trailtech for £80. Backlit ,revs ,trip,engine temp

I got mine for £4 from tesco, handy that it stops at 64mph so i never speed, shame its not got a back light tho.
There great for town use, i wouldnt be without one these days.
Cheers
Ian

Can anyone elaborate on how you fit and calibrate these please. Last one I had on my mountain bike needed a magnet around the spoke and I had to tell it the wheel size. Would it be the same and if so where do you fit the magnet.

Yeah you need a small magnet, i stuck mine to the disc carrier (you may need to check wheel balance), and made a small bracket for the sensor. Then all you need to do is measure the rolling radius of the tyre and input that into the compter.
If i get me bike back from me mates later i take a pick for you, tho it be on me nc30.
Cheers
Ian