Thought I'd come back to report that the icos 33080 is now working again.
It was rather worse than it appeared.
When replacing the fan harness, I looked for where the cable should be attached to the case, and noted a plastic clip top left of the case which had been charred.
I also found the long hexagonal central pin holding the burner on was loose. These were obviously of concern, so I reluctantly unscrewed the burner top.
Good job I did - fully one third of the box underneath the left of the burner had melted away, causing a gaping hole at the rear that was completely out of sight, charring the interplate, knackering the burner gasket and even causing a positive feedback loop by heating the gas inlet pipe.
It had been caused by the rear insulation panel tilting forward.
Unbelievably, the side panels which retained it in position had chamfers cut which enabled this movement to happen, exposing the aluminium casting directly to the combustion region.
So, not the PCB, not the fan, not the harness, but poor insulation design causing subsequent failure of the burner box, harness, fan and PCB.
I later accidentally stumbled on
this thread
(with picture) on another forum which described pretty much the same issue - but mine was rather worse.
It even looked like one or two of the vanes in the HE casting had possibly started to melt.
I sourced a complete HE+burner replacement, plus all the gaskets, and a Gas Safe engineer who was experienced in Ideal boilers who has put it all back together safely.
I'll start another thread about how to identify an alternative boiler to replace this one at an appropriate time.