Ken dont buy lobs mate you can get thousands for free. Wait for a damp evening,take a torch and a bucket to the nearest playing field and if you tread lightly you will see the lobs sticking out of their burrows. Take hold until the lob is free of the burrow,a gentle pull is all thats required.
Keep them in good fettle as Eddie says in some barely damp shredded newspaper in a good container that allows air to circulate but doesnt let the worms escape. You can feed them with a few vegetable scraps or even porridge oats to keep them happy.
Dons right about buying, they are cheaper to dig. But when you are on your way out the door, if you dont have any in, you have no choice, I get mine from my local shop!
I keep saying am going to start collecting them, but not got off my arse basically.