Firstly, thanks for all the help and advice, I really appreciate it.
Secondly, the plan as I see it, which would work for us:
Slowly increasing the milk quantity and decreasing the milk temp. I'm going to try increasing each feed by 50ml for this week and see how much they take, and hopefully have it only lukewarm (feeding it at 40 C at the mo), then increase slowly every 4 days or so until they're on about 1.5L a day (as recommended). I'm also tempted to try a bucket feeder (it's only about £15 at Mole Valley, so worth a punt!) and see how they get on with that.
I'd like to keep the hay available, but will also start adding creep for them to nibble, but monitor how much gets eaten daily. Start with about 250g in a trug and see how much goes.
A bucket of fresh water, and again monitor how much they drink.
Aim to wean around 6-8 week mark, depending on how they get on.
Keep them in the shelter for the time being, looking to make it a bit bigger as they grow.
So that's it! Any thoughts? I know any plan never survives first contact with the enemy, so here goes!!