When the weather outside is so cold the mains water supply is coming in close to freezing you certainly notice when the boiler stops heating your water. At first I thought it was just cutting out do to over-pressure in the heating system (having topped it up previously). However on closer inspection we were seeing leakage.
The rather excellent Plumbing Birds came out and spotted the first of a number of problems. Vaillant had issued a service bulletin to remind people to replace the old silicon seals with a newer more heat resistant graphite one. This hadn't happened in our case and this had resulted in a fire which had melted the air intake of the boiler (but luckily nothing else). However we still have a leak coming out of the heat exchanger which accounts for the slow drop in pressure that has been observed in our heating system. That part is on order so hopefully it will be fitted before Christmas. In the meantime the leak is slow so we at least have heating although I'm monitoring the boiler for any sudden build-up in leakage. At least we now know why we found some of towels in the airing cupboard where damp.
It looks like I'm getting a bonus this year for excellent performance both from myself and the company. I guess I know what I'm spending it on now.