Posted: 03/05/2015 at 11:39
In the first instance I would use this http://www.pestcontroldirect.co.uk/acatalog/Rat_Control_Pack.html
It worked in this garden when we first came here and we've not had another infestation.
The poison used will work on rats that have developed a resistence to Warfarin.
Also check around any drains, manholes etc for cracks, holes, ill-fitting covers etc to make sure that they're not getting onto your property in that way. If they are it may be useful to contact your water company - if your drains are on a shared system they may well be responsible for repairs and dealing with the rats.