Go to Rentokil and buy somke of those black plastic rat bait stations. You can buy the blocks for them in bulk or in small packets but Rentokil won't sell them to you. Use a mixture of green blocks (multi-feed) and blue (one-feed kills). Green are grain based, blue are chemical (and residual) and you'll need the green to attract them, or use Toblerone pieces with the blue blocks. If you have native animals or cats around that eat the rats don't use the blue baits, use only green. If you keep bait up to these you won't have rat problems again.