An exact replacement for the original part, this can be replaced by anyone with a bit of common sense and the manual (downloadable) - isolate the boiler, turn off all the valves and use the drain cock to get the water out of the boiler (use a bit of tube to drain into a bucket or similar). You won't get all the water out, so have a shallow tray and a towel handy for when you unscrew the old valve. Worth getting some silicone grease as well, smear a bit on the seals.
The included tool is a bit flimsy, but does the job for unscrewing if you are careful, and the flimsiness stops you overtightening the new one.