Thanks all, much appreciated!
Here is part 2 of what promises to be a bit of a saga...
After remeasuring, it turns out that the length from the front edge of the pilot's cockpit to the start of the rear decking is correct, so the problem is that the observer's cockpit is too far aft. To remedy this, I squared off the rear part of the cockpit opening and glued in a couple of fillets of plastic card. They were left to dry overnight and I then used the Miniworld Scarf ring to mark where the opening should be and clipped out most of the plastic with sprue cutters.


A session with a small curved jeweler's file completed the major work there, it will be refined when all the major changes are done and I separate the fuselage halves again. The insides will need some thinning out!
The next step was to work out where to remove the rest of the excess length. It turned out that the turtledeck was too long, so I marked the cut at the rear of the observer's cockpit, just where it joins the decking. I marked 2 lines with 1mm masking tape, the 2nd one being the rear of the 2mm piece that needed removing.
I cut between the two lines, making sure I didn't get too close to either of them...

I was left with the 2 halves, each with a piece of masking tape on, and simply sanded each one back to the tape..


The two halves were then reattached, making sure that the rear part was raised to the level of the front - the step between the 2 that exists on the kit should not be there...


This now leaves a couple of small issues to sort out...the fuselage now has a step at the bottom, but since it was far too deep anyway, the front half will simply be sanded down to match the rear, sorting both the step and the error in depth at the same time. I will also need to sand the sides slightly to match them but that is a very small lip and won't really affect anything.
You will also notice a line of "stitching" below the cockpit which should match the line along the bottom of the rear decking. This is also not a major problem as the cockpit sides are too low and need to be raised. This line of stitching should actually be a straight line which runs along the bottom edge of the rear decking, (correct on the kit!), rearwards to the tail, and forwards, just under the cockpit sides, to the front. All I need to do is remove all the moulded stitching and add some PE stitching in that straight line - it does match up as reassembled! The upper surface of the tail, just aft of the decking will need to be sanded down as it is too deep (top to bottom) and kinks upwards, so it will be straightened to match the above mentioned stitching line.
So that, basically, is all that is required to correct the fuselage - not as much work as I'd expected! A lot of sanding, but not too much surgery....at least I hope that is all of it, it certainly matches the plans pretty well now!
As mentioned there is still a lot of thinning etc to do internally.
Thanks for looking in!
Ian