Thanks for the updates all. I've recently been using a 50/50 aldi clumping litter and the Sainsburys eqilivent to catsan which worked quite well.
However last week I bought a bag of the crystal litter to try (I have two trays and only put the crystal in one in case the cats didn't take to it) and am very impressed thusfar. The cats seem to prefer using it too. I'm not sure that it will last a full four weeks as planned - I'd be happy with just two weeks. It was only £1.99 from Home Bargains for a 3.8 litre bag if anyone's interested.
I hope I don't come across as a lazy cat mother and not wanting to do frequent changes - thankfully I'm not squeamish and it doesn't bother me...my main concern for trying to get stuff that lasts is that me and Mr Lil Scruff might go on holiday later on in the year and rather then take the cats to a cattery my/his mum will come and feed them and I'd want to minimise the more unpleasant jobs
Cheers again all,
S.