The replacement cylinders looked OK as far as I could see, and were duly fitted. However I could not get the brake drum back on - too tight. I then decided to clean up the inside of the drum which was serviceable but not perfect. After quite a while and much effort - drum would still not fit.
The two leading shoes on the font brakes are held in place by the wheel cylinders, expansion of the piston in the cylinder pushes the shoes outwards and applies the brakes. I therefore decided that perhaps these new cylinders were a little too long between the shoe abutments, dismantled the brakes (again) and measured very carefully. Sure enough, one was about 0.5mm longer than the originals. Much careful work with a file and all looked well - I filed the slot on the opposite end to the piston, the slot which takes the trailing edge of the shoe. All fitted back together and guess what - drum would still not go back on without seizing.
Dismanled again and much measurement, comparison with the good side (RHS), studying of parts books and manuals. I could not see anything wrong.
Reassembled the brakes. Had a look at the slot I had carefully filed to get within tolerance, and to my amazement noticed that the brake shoe was not sitting in the slot. It was being held by the edge of the brake cylinder casting, preventing it from seating, and hence was way too far out and touching the drum at all times. Witness marks on the underside of the brake shoe confirmed this contact, I decided not to dissassemble again, and took a small grinding wheel and removed the offending edge of the casting, And all was well!!
The image below shows the extra unwanted material on the casting (circled - and clearly different to the original wheel cylinders) - this is what I removed. The arrow shows the slot in the wheel cylinder which holds the trailing edge of the shoe. That additional material prevented the shoe seating here.
What a way to spend Boxing Day!!
Ian F