A quick update,
I’ve just spoken to Ian Porter at Ian Porter sales. He couldn’t have been more helpful.
Mk1 grp boats have a 5/8″ mahogany tabernacle, bolted with 3 off 1/4″ or 6mm through bolts to the foredeck. Phew, looks like the repair should be just a case of beefing it up with fibre glass under the foredeck.