Hello James and other amel owners,
I have read this topic with interest because my Maramu 1985 (single spreader, non furling, Isomat masts) seems to have the same issue as your boat. I would say even worse. I followed up on all the tips that were given eg, check the compression posts in engine bay, check for cracks in fiberglass in and around seats and complete mast step other than those present. Checked the base under the compression posts, rack of the masts, tension of the rigging etc.
The rigging has been totally renewed by a rigger in Hyeres, Fr and very familiar with Amel boats. After the sail to Holland I tightened them slightly following instructions of the same rigger. Anyway, the cracks at the mast step on starboard were already there, but smaller.
One thing I found is that the base where the compression posts rest upon are made of steel and not wood.
Of course they are fully fibreglassed so hard too inspect properlybut there are no cracks or deformation visible.
You can imagine I am very interested if you have sorted the problem and how. Also I would like to learn if the steel base of the compression posts is original?
I will post photo's in album "Mizzen maststep deformation Maramu"
1985 Maramu, Lagos Portugal.