Thanks all for the advice.
Fixed it! The rubber seal had jammed the catch but also the mechanism is broken, well pressing the button does not move the catch.
So I prised up each end of the cover in turn and then with a scalpel taped to a thin piece of flat bar cut away the rubber. But still the catch was barely moving. I tried various things but the solution was a piece of string threaded around the catch and a stout pull. I am fortunate in having some extremely strong string made from kevlar; my guess is garden string would not do it, or maybe use bicycle gear cable or similar. I will replace the lock with the old style keyless handle, more leverage and one less key to loose.