Oh well - as you predicted, the one that we "potted on" into the grow bag finally found it's feet and we have had at least four big cucs off it so far with a couple more babies on the way when i last looked. The one in the pot has floundered after producing one cuc. It looks absurdly pot bound now - I just hadn't appreciated how big they would get.
I think the light situation isn't very good for them at all. Next year in the new polytunnel I am going to plant seeds directly into upturned gro-bags so there will be no risk of root shock. We will be using reclaimed peat compost - a friend can get bulk discounts. I won't put them "behind" the toms as I did last year. We'll probably only grow on plant in a good position as I think that will probably supply all our cucumber needs!!
The cucs themselves have been fantastic. Not nearly as "wet" as shop bought ones and keep MUCH longer.
Thanks for all the advice