Thanks Jim and Rocky.
Problem solved. I tried the 'proper' size Schwalbe tubes again, and, with the tyres fitting much more easily 2nd time round, they went in OK with no risk of pinching, as the tyre went over the last bit of the rim more easily - still tricky but OK.
First impressions of the new tyres on a short round the block spin? Very light and fast, bike more lively feeling.
So, now I have a pair of Conti tubes that the bike shop insisted I needed! 26", up to 1.3 tyres. Put in a tyre and pumped up to pressure, but not ridden on. Great care taken to avoid pinching. No other bike in my family they will fit in.
Free to a good home, in recognition of the advice I have had from the Forum. Anyone interested pm me.
Lewis