nobody has looked at the fact that the axle maybe sitting ever so slightly proud of the frame making any q/r attach itself to the wheel rather than the frame
I am using modern wheels with their q/r's in a 40 year old frame with its horizontal drop-outs with no issues
a sortof googly image to show what I mean:
Yes Robbied196 suggested I check this - which I did last night. Each axle end has about 1.25mm before the end of the dropout so I think it should be fine?
It definitely felt like the skewer got a good clamp. I removed some of the paint from both sides of the dropout area (specifically where the skewer "teeth" will contact).
I also ordered a set of Shimano internal cam skewers. I also ordered a couple of Sturmey Archer serrated washers with 9.5mm dropout slots. Only a couple of quid so might give them a go too. I assume they are around 1mm thick, so that would give extra room between the axle end and outside of the dropout.
So I am hoping that the new skewers combined with better contact with the dropout, and the serrated washer if need be - should be enough to fix the problem.
There is nothing more disconcerting than cycling with the feeling that your rear wheel could unattached itself at any moment - well, except a front wheel I suppose. I like my wheels to stay in place!
Thanks for all the help I will update you when the new parts arrive.