why dont you just remove the fuse from the fused spur before you try disconnecting the N and E and then monitor the behaviour of the mcb marked downstairs ring that keeps tripping........i would try this first.
Removed fuses, turned on MCB, tripped 15 minutes later
Is the RCD tripping..
Both MCB for ground floor circuit and RCD trip yes
Then unless its ancient, the switch will be double pole. The back of the faceplate should indicate that. So switching it off, will isolate the electric ufh from your RCD.
Oh it's been switched off for a couple of years, and i did just remove the fuses and test but 15 minutes later the MCB/RCD both tripped. So maybe it's not the UFH at all.
A N/E fault cannot possibly trip the MCB.
An earth fault can cause a dead short
Conflicting information ?