Neil,
Unbolt the seat and remove it, its straightforward, 6 UNF nuts and bolts. Then ease your carpets up and start looking at the rocker to B post union ( timber sill construction). the glue can pop and the screws loosen at a joint and you will need to find out whats going on if that's the case. Its a common area for water leaks and the wet sits behind the carpet covering that inner area and the ash frame can start to suffer. Yes the chassis' do flex but generally the ash body frame is pretty robust and can accommodate this movement. It's on the near side too and this is the side that sits in the wet found at the road edges. If you do have water penetration try and ascertain where it's coming in, It might not be obvious and could be finding it's way there from the most unlikely of places from the bulkhead/scuttle back!
A good adhesive if needed is a polyurethane glue like Gorilla glue it sets better in contact with water and expands to fill any voids caused by damaged timber. use fairly sparingly and syringe it into tight joints. masking tape any bits you don't want glue to get onto as it expands .... you won't get it off! I have drilled 3mm access holes into the joint before if its a tight joint and hard to get glue inside the joint. just drill to a couple of mm past the joint faces and squirt in the glue then screw in a self tapper, short with a flanged head. This stops the glue pushing back out and helps force the glue into the joint space as it expands.
There's always a certain level of detective work needed on a Morgan and often the cause can come from somewhere unexpected so start looking around, It might be that the chassis took a shock from something like a speed bump or pillow and you have suffered a stress crack, jack the front of the car, then the rear and see if it imparts any movement at the fault line, if it does try and ascertain what else is flexing that might cause the movement at the sill then ask yourself if it's a normal level of flex and what is allowing the timber frame to move enough to crack the alloy cladding.
Good luck and enjoy, there will be an answer it just takes more time sometimes to work it out!
