cbosleeds has a good point about invalid code probably not being the problem of reduced traffic from the other search engines - particularly as you're in the UK - UK search market share is dominated by Google, and the others only get a tiny slice of the cake anyway - about 2 months ago we did a survey of some of the heavier traffic sites we look after, ones that are ranked well on Google and on Yahoo/Bing, and the average for organic search engine driven traffic was 88% Google, 12% the rest, so just be thankful you're getting traffic from Google!
Most search engines are pretty tolerant of code with errors in it unless it's really broken, but I'm always in favour of cleaning up errors if possible, so to come back to things to help you...
Most people here have pointed to the W3C site, but there are also browser based tools that help you identify problems with your existing code:
Opera with the optional debug tools (that link out to W3C validation tools etc) plus Dragonfly
Firefox with the HTMLValidator Add-in + Firebug
But for me the best HTML reference is http://www.w3schools.com/
Covers pretty much every technology as well as HTML. Searchable, continuously updated to latest specs (and browser compatibility), with good explanations and code examples and it's FREE! Stacks of tutorials as well if you need them.