OK, the banging, when you run the bath. When this happens next, have a look at the digital temp gauge on the boiler, it'll prob be much higher than it should be, prob around 90-95 deg. Also the boiler will prob be banging as well. I think the problem is you heat exchanger is clogged up with gunk. The water passageways are tiny, so as to be most effective. Now when you call out a plumber, he will probably tell you that you need a new one, as you can't clean them. This is 100% wrong, you can clean them very effectively. When the heat exchanger is removed, then fill a bucket with clean water, and add some 'spirit of salts' into the 4 holes of the HE, filling it up, then place the HE in the bucket of water, and wait for the action. Much fizzing, popping, and general merriment, for 10 mins, then swish it around, then rinse with clean water. The old water will be black, and full of bits. Had mine done twice now, costs about £45 at go, ( at least 2 years apart) to take out clean and re fit, but that's prob cheap.