Hello again and thanks to everyone for the suggestions,
I'm not planning on taking the lads too far, nor for too long to start with. The idea is to wean them into things slowly so they (especially the younger one) get used to being away from mum. One or two nights in a large wood a few miles from home or a weekend here and there at a local scout camp should suffice until they are a little older.
There may be the odd time when I'll be taking both lads alone, but sleeping arrangements for 90% of outings are likely to be 1 x adult + 1 x grandson per tent. This arrangement could see the Banshee 200 and Gelert Solo being a little too small, but the Banshee 300 and Gelert Mongoose look like being worthy of further investigation, as do the Tesco Ultralite bags.
Thanks again.
Alan