iPhone is now sorted - cost £25, on top of that I've paid £1 for a charging cable and ordered a Freedompop SIM, they seem to be OK if you don't need a huge amount of data a month. That costs £6 including activation - I could have got an extra sim for 1p but I completed the process before I realised that it wouldn't have been a bad idea to have a spare. Too late now anyway. It appears that the phone isn't locked, which is one less thing to worry about.
Thanks to coth for her help with this, it's greatly appreciated - sorry for wasting your time.