Found enough time/energy today to have a first look in the engine bay and do a compression test.
Everything is quite grubby, but at first look seems more or less complete/intact.
The leads have a two part insulating plug at the spark end. On cylinder 1 (does Volvo count from the front?) Is missing one of those parts and so doesn't attach well.
The Plugs were barely more than finger tight, but at least came out easily. All were in similar condition.
Compression is nice and even - not sure what the ideal number is for these, but I'm not getting upset by 150-odd!
Will keep digging. Curious about the clatter coming from under the rocker cover, so will check there next.
T.