My RCD keeps tripping and I can't figure out why. But what I can do is
reproduce it !

I've got a pond feature with the pump wired into a waterproof switch.
The feed for the switch comes from a waterproof DP FCU. The feed from
this is taken from a socket inside the house.

When I switch the pump on, everything works just nicely thank you. But
as soon as I switch it off, it throws the RCD.

The RCD in question is on my consumer unit. It basically covers all
but the lighting circuits.

I've unplugged all electrical appliances (in case of the cumulative
leak issue) to eliminate that. I say all, I've not been able to
disconnect the shower or oven. One thing to note, running off the FCU
is another filter/pump. It needs to run constantly hence it not being
on a switch (albeit the FCU has a switch on it which would be
permanently on).

Any ideas why this might be happening?

I'd have thought you'd more likely to get a trip when turn an appliance
on, not off !

Thanks, Andy