Thanks for all your replies, though I haven't had much time this evening to research them I willl do tomorrow.
I have found a local bike shop with a good reputation and had a brief chat with them, they showed me two bikes in store that after a chat they thought would suit.
I have found a child seat that though expensive clamps onto an existing rack and is a good size so should last my son about 3 yrs if necessary.
Discussing this seat it became apparent that it wouldn't go onto a rack with a battery mounted under so we looked at step through with tube mounted batteries. They showed me 2.
One was a bergamot and the other if I remember right was a scot. Both priced at about £2000.
I hadn't really considered a price as until i reaserched I really didn't know how much I would need to spend on a bike that would fully meet my needs.
I guess I was hoping that I could pay a little less, say 1500, but I could raise to 2000 if it was really necessary.
Can anyone comment on either makes, sorry I'm not sure the models as there seem to be many around that price range.
Thanks again.