Gavin called me because his boiler was locking out and he had no hot water. The problem was a scaled up secondary plate heat exchanger that was stopping the water from circulating properly. I swapped it out and left him with hot water again.
The boiler kept locking out because the water could not circulate around the old plate heat exchanger. It is a common fault on older systems where scale and debris build up between the plates, and the easiest symptom to spot is hot water that never arrives even though the heating seems fine.
I removed the hot water pipe to get the old plate out, fitted a new secondary plate heat exchanger and put the pipework back together. Once the system was refilled and purged I fired the boiler, checked the combustion and made sure both heating and hot water were working properly before I left.

Tell me what you need and I will come and look at it. You get the price before I start.