Just to follow up, in case anyone finds this post doing a search, I fixed the problem.
I replaced the relays, as it seemed the most likely cause based on past experience, but that didn't work. I then followed the fault finding procedures in the service manual, and identified it as a failed fan...
My 28i Junior delivers hot water fine. It switches off and back on at intervals, but only for a few seconds, so not enough for the water temp to change significantly.
The CH is very erratic. Sometimes it runs OK, but most often the fan switches itself off a second or two after ignition and...