Poison and traps is the way to go.
The poxy stuff you can buy from B&Q and the likes is useless though. You're best off ringing the professionals as they have access to the proper nasty stuff.
As said - remove all sources of food first though and also play hell with burrows etc as rats are neophobes and will bugger off if their routines get disrupted enough.
Last house had rats in the roof (coming up a drain pipe) and they were a right pain to get shut off. Lots of poison and lots of disruption worked in the end.