OK, so I ended up having to drill the final screw out, it was just too corroded, but it did come out nice and easily and my drill went through dead centre with no damage other than to the screw :)
More dirt suggesting a possible flood, but all very dry. I'll need to get everything in hot soapy water to get all the crud off.
PXL_20260804_081917500.jpg
Despite the presence of Nile silt, I think the mainboard looks OK-ish:
PXL_20260804_081930933.jpg
And I think this is because the scratches on the case suggest it was stored upside down, so whatever washed through the case caught in the top and not on top of the board.
Unfortunately this meant the bottom of the board caught it:
PXL_20260804_084850591.jpg
Edge connectors are quite corroded, however they are still there and I believe they will clean up fine.
One last thing is the keyboard membrane. it actually looks OK. Wether it is or not is another question, but I've got a fair bit of work to do before I can verify that.
PXL_20260804_081942183.jpg
Guess I need to chuck the case in hot soapy water and give it a good old soak and blast all the dust off the mainboard to get a peek at what it looks like before I decide how to clean it.
One thing. It looks way bigger in the pictures than it is in real life :)