I understand it is a motorised, 3-port 22mm valve.
honeywell and drayton are the more favoured brands, but assuming your Myson has lasted many years and given good service all the brands will fit. There might be slight differences in length.
Before disconnecting the wires, draw and photograph the connections, taking great care to note which colour goes to which terminal, and verify that the colours are the same on your old and new motor. If different (unlikely), the manufacturers will tell you how to compare.
As an amateur, you will need a set of adjustable spanners big enough to grip the big nuts (you can get low-quality ones in suopermarkets that will be adequate for a few jobs) and I would strongly recommend that you drain the system first, so you will need to ie up the ball float in the feed and expansion tank, bale out all the mud, and wipe it clean, before opening the drain-cock.
As you will be draining and refilling this would be a good time to give the system a chemical clean and on final fill use new inhibitor. Sentinel X400 cleaner and X100 inhibitor are very good and will cost you about £16 each. The boiler will have to be turned off until all work is complete, but assuming you have an immersion heater in your cylinder, this will be no hardship at this time of year.
http://www.myson.co.uk/static_files/my/media/downloads/MPE_322_Installation_sheet_Sept_2009.pdf
p.s.
see if you can take the motor head off and operate the spindle by hand. If you can, and it works, just buy a new valve and swap the heads. Keep the rest of the valve for when it wears out.
