I took my bike to the local bike shop. He didn't narrow down the problem, just gave me a quote for £150 (£200 inc. labour!). I'm going to replace the crankset, chain, rear sprocket because they're old. My crank is actually a little out of alignment and on each rotation, the chain touches the front dérailleur, but this isn't the cause of the clicking, which I still haven't sourced. The click has now got so frequent it happens on each rotation, sometimes twice. The noise is amplified through the frame, so I'm not sure of the location but I'll replace these parts as they aren't too expensive, and if needs be, I can still return them.

Is there guides on this forum of how to replace parts properly? I'm not new to cycling but I'd just like to make sure everything is going on correctly.