Hi Gavin and Helen!
Re: feeding mom cat - you can go to Newport industrial estate and find Jolies per shop - have a look for Royal Canin Baby Cat first age - its packed with everything necessary for mom-to be and for first couple of months of kittens life.
Her birthing box looks phantastic - but you might need to do something with the lid - just in case if you will need a quick access to mom in case of emergency.
Dont wont to frighten you, but sometimes kittens born wrapped in the sack and first time momcat might not brake it quickly enough; so baby might get cold and die or suffocate.
If you will see it happened - just rip that sack and gently wipe baby's face.

__________________
"Do not meddle in the affairs of cats, for they are subtle and will pee on your computer." --Bruce Graham
There are no rules, rules are only for people who fit inside a box, if you want to think out of the box there is no hope... Life has no Box. (Tony Howells)