When using the search facility, if the results show you a thread which roughly covers the topic in question but is over 6 months old and you want to ask another question based on the original topic (you with me so far...?) which is proper messageboard etiquette? Bumping the old thread, or starting a new one?

The reason I ask is that I have noticed a few bumped threads being closed every now and again, but then also noticing the aim of the mods to keep the boards tidy, so just wondered which was the preferred method - keeping an old topic going, or starting a new one?

IMO Its better to start a new one. You will get more focus on your question then because it will be in the first post and not the last.

Generally, when you have a new question to ask, start a new thread. However, if there was a gigantic topic about, for example, what weapon loadout is better, etc., and you have a NEW opinion to toss into the fray, it's okay to necro-post. It's good that you're using the search function; many people just start up new threads when there've been tons of threads covering the same topic. So, basically, use your judgement. If it is something close enough to brand new, go ahead and start a new topic; if it is something old with a new twist on it, it okay to post in an older topic so long as the topic is a big enough one.