In the last couple of days our fridge/freezer has started tripping the RCD, but the circumstances seem a bit strange. 

I've tried the fridge on a different circuit connected to a different RCD and it still trips.

The fridge/freezer trips at or very close to 6:30 each evening. It was exactly 6:30 last night and the night before my wife said it was close to 6:30. 

Plugging the fridge/freezer into an isolating transformer appears to reset the problem. After running connected to the transformer for 15-20 minutes I can reconnect it to the socket directly and it runs fine.

Any ideas on what might be causing the problem? 

I am assuming the fault is with the fridge/freezer rather than the house wiring?