If whoever did the repair can't reinstall OS X 10.6 Snow Leopard, and you don't have a backup, you can buy a new DVD of Snow Leopard from the Apple online store. Either call them (in the US it's 800-MY-APPLE) or you can order online too. That doesn't come with the iLife applications, however.
I have a dual boot system with OS X 10.6 Snow Leopard on one partition and OS X 10.7 Lion on the other. That allows me to still use some PowerPC-based programs that will not run under Lion.
Call the online Apple store at0800 048 0408 . It costs $20 in the US which should be around 13£. I did read somewhere that if the online Apple store in UK doesn't have it, to call Apple Support.