We recently had an extension built and all has been signed of and is fine. however, there was a power cut last weekend, and now the RCD circuit we have the ringmains on keeps flipping

Any ideas?

I would unplug every thing on that ring and see what happens. if it still trips then the only active component is the RCD, so i'd change that, if that doesnt work, you then have the joy of trying to find the short. Note where I said you, i meant an electrician as you are not allowed to play with distribution boards any more.