Ok, I think iv noticed the issue.
My CU is has 2 RCD's protecting 6 circuits each.
The circuit that kept blowing: the neutral wires were wired Into the neutral bar for the 2nd Set of RCD's (hope that makes sense).
This would explain why both RCD's were tripping.
That's a very good point!
Iv just tried it all again.
The socket tester itself didn't trip the RCD's. But when I plugged in an appliance and switched it on - the RCD tripped and the internal Fuse on the socket tester is now blown
I bought a socket tester. It shows no faults on any of the sockets.
If I turn the MCBO on, and leave the 'ring' alone, the RCD does not trip.
If I turn on any of the switches (nothing is plugged in), the RCD Trips.