All fixed!

Reassembly with the new board was straightforward with the exception of the display ribbon connector. Seems I missed the fact that it has a locking tab when I withdrew the ribbon during disassembly. A black tab that sits across the top of the connector pops up when it's edge is located and lifted with minimal fingernail pressure, or using a suitable plastic lever (not metal, that could result in damage). Once this tab is raised and released (it remains attached, so no risk of losing it) the ribbon easily enters or leaves the connector. I found it best to do the ribbon bit when all other cables are removed from the board, giving maximum ease of access.
After replacing the cover and connecting speakers and power the standby light quickly responded to the arrival of power.
When faulty the unit would display a familiar blue graphic screen for quite a few seconds before being replaced by the standby light, but the unit was then totally unresponsive, apart from the production of smoke & smell.