Rob,

spoke to a colleague today about this. He said any alterations to my central heating system would have to be checked by a Gas Engineer who is "Gas Safety Certified". This is a new scheme in the UK, and it'll likely be expensive to call out an engineer to do this. He also mentioned that your idea might cause the outlet gas to condense as it flows and cools around the loop of tubing in the garage prior to being exhausted. This would obviously not be a good thing at all.

I'll did a bit more looking into it, but it might not be all that feasible