That definitely is not happening here. Boiler starts up, runs for a minute or so accompanied by increasing clanking from the feed or outlet pipes at the top and then shuts down. Overhead light does not illuminate.
As a follow up I decided it was time to call an expert in. Quickly diagnosed a blockage in the system and a failing pump. There was an almost complete blockage just above the pump requiring a short section of pipe to be replaced. Also the three way valve was corroded and the vent was similarly knackered due to corrosion. All parts replaced and now working again. Also moved pump further up as it was poorly sited (see earlier pics) with and fitted gate valves. Has advised a get the system flushed, hot water header in loft looked more like sewage than water
Thanks for the update....so despite your thread title, no repairs to the Baxi required in the end then . Funny how the boiler always seems to get the blame, but in this case it was everything but . Odd that the overheat didn't trip though.....
In case anybody is interested the chattering relay problem which will eventually burn the relay contacts out is caused by failed decoupling capacitors on the supply rail. From memory they are the ones rated 47uf 100v and I think there are 4 of them, replacing these will fix most Baxi solo 2/3 control panel faults.
For safety reasons the pump relay is configured to always supply mains power to the pump unless the relay is activated to turn it off therefore is should always fail safe preventing over heating.