how do i replace the diverter valve in my boiler?

Well if you've never given it a go I'd advise you to get a professional my friend. :)
If, in hot water only mode, you're boiler does not light up then generaly the issue lies with the diaphragm and not neccesseraly the whole diverter valve. You can purchase a diaphragm or even a 'service kit' which will save you loads!! However you need to know what you are doing! :(

Diamond Gas
September 2008