The wiring goes like this, for most current valves which could be used as a replacement...
[code:1]Switchmaster ................... Honeywell etc
Blue ............................ blue
green/yellow......................green yellow
yellow............................white(poss brown if no white)
white.............................grey
orange/ red link together.........orange[/code:1]
A new valve will cost £50 - £100 depending. There will often be a need to alter the pipework, though you might be lucky. Depends on the location exactly. It'll probably be drained to some extent.
Total cost could be in quite a wide range, but £200 might be typical.