Hey thanks for all the responses to my post!
I have done a botch fix as I described by pouring epoxy into the mast base hole with a plastic tube stuck down the drain hole which I then removed when the epoxy reached a toffee-like consistency.
I first let some of the epoxy drain through the split timbers into the outboard well (drips caught on a plastic sheet that I spread there for the purpose), then I pushed the cracked and now glue-infused ply back into place which was very easy and held it there with firm ish pressure from a car jack while the epoxy went off. The next day I put the plastic tube back in and added a bit more epoxy into the bottom of the hole for good measure.
Then on Friday I rigged up and went sailing so for now as least the fix seems to have worked well enough and it is invisible unless you look upwards through the tiller hole in the transom or peer into the mizzen mast base hole with a torch.
Fingers crossed
? and thanks again for the help and advice.
Final thought: I will try to get hold of Andrew Denman and mention the issue to him because it seems to me that this is a bit of a weak point in the design/construction that he may want to address before making any more boats…. but if anyone else has his or swallow yachts’ ear perhaps they could draw attention to the issue in case I don’t get through.