Got exactly the same problem with my 21, all the people I have been talking to, have said spilt the hull, as this is how the transom was originally fitted. The problem I have, is that the joint was glassed over on the inside, so will be hard to find, Steve Baker made Phantoms light by not using much fibre, so I could sand through the deck trying to find the joint.
How shown the boat to a couple of companies and have decided to go with Ricard at Romney Mash Marine, thought £1500 was a very fair offer to undertake the work. Goes there once I have striped it in November. Will post updates.