Denso is correct regarding hijacking old threads. Saves a lot of time wasting reading old posts. Back to your problem. The valve is perfectly repairable with service kits and I have never replaced the whole valve. Replacing the whole valve is the easy way out and the most expensive IMO.
If it's simply no hot water, caused by a holed diaphragm, then a £10 service kit and 30 mins will fix the fault. There may be other reasons, but the diaphragm is the most common.
If it's simply no hot water, caused by a holed diaphragm, then a £10 service kit and 30 mins will fix the fault. There may be other reasons, but the diaphragm is the most common.