Thanks for the replies you two
i think i'll give the zoo plus trees a miss!
Lesley, thanks for the link, think they're a bit too much at the moment, have just had to hand over my life savings for car insurance so am about as skint as can be at the moment.
Have been rethinking things and wondering how to give them some fun things to climb on a budget and i'm wondering if a series of shelves on the wall coupled with a few wall attached cat hammocks might do the same thing but be cheaper and stronger...
Will have to chat up my mum to come and drill lots of holes in the wall for me.