It was the stainless caps on my transom that went first and we think that the majority of the water got in from the top, the thin slice of stainless Avon had fitted had almost disitigrated, after my transom was replaced the boat builder formed a thick edge of epoxy and my mechanic had a thick stainless steel cap made which rapps well over both inside and outside edge, not only does it look the dogs now its doing its job.
let us know how you get on with this, though its still worrying that these things are happening to relatively newish boats !!