Thanks for all your replies, don't worry cat flap is locked, I will NOT be letting either of them have kittens!!

I spoke to the vet and have booked them in for 18th March, think I might weigh them tonight, although that could be tricky as they don't like sitting still!! They are both the same age so will get it done together, have booked 2 days off work so I'm around as soon as they are ready to come home and then with them all day Fri and most of the weekend.

I guess after recovery the fun really begins when I have to be a brave mummy and let them out without me!