DJEs warning was well timed! Thanks again.
The frames are marked with lines to ensure the frame is assembled square (at least that is what I felt these markings are for). They had glued up nicely & it all looked square & measurements 'matched', no curves, odd gaps etc to worry about.
After a lot of dry attempts to match skin to hull frames, I decided that I would attach the glacis plate front section of the upper skin first, ensuring the hatch opening was 100% lined up.
I'd measured the distances with calipers & things seemed OK but...
Recalling my clockwise method of skin attachment from my BTR once the glacis plate had dried I started trying different positions to tag the skin & test alignment.
The result was attaching first one side, then the other. Not doing this (ie gluing the top down first) would have thrown these out in relation to the frame. (Re - DJEs comment about the 'bumpers').
The end result is the 2 sides & glacis is 99.99% perfect fit. The top - not so much! :(
Starting at the back of the turret ring I had a tetrahedral shaped bulge/ripple, the high point of which was at the rear sloped plate.
I decided to do some surgery, cutting through the turret ring to the engine bay & then from the engine bay to the back. The 1st cut is central & I took out about approx 1-2mm. I expect this to be mostly concealed by the turret.
The 2nd cut is off center & as it will be concealed under an upper cover (#40) I haven't (yet) cut out the slim triangular section to make the cut edges flush & skin flat. I will see how much is concealed or flush before trimming this for a clean fit. (See attached schematic)
The central point (for #39 & 41) of this skin remains centered according to the calipers, but I had an overlap of <2mm on the one side once the cut was made.
The top part of the upper skin is rippled in an uneven way, so I will need to made a few extra cuts. The turret ring opening will surely need some cutting as the circle is now not 100% aligned with the opening in the frame at the back edge.
Thanks to DJEs warning I aimed to keep the openings square from the start. As the pic shows it is pretty much square now & with a bit more twisting & glue I am pretty sure it will look Ok at the end.
I have come to see this kind of work-around surgery as a good aspect of paper modeling, forces me to think & experiment more & is more art than assembly. It is a good thing to learn to think on-the-fly, even if these things catch us out sometimes. :P