I'm taking stock of this whole affair. The first thing is that the original oxygen and moisture-scrubbing catalyst last for 30 years! I know people who insist on changing theirs annually. So although the box is messy as hell it's actually been well looked after.
There was one serious wrong turn - we mis-diagnosed some of the symptoms and wasted £300. In the end the part that needed replacing cost £15 and a little soldering. Plus the replacement catalyst that was lying spare in a cupboard. 1/2