Question Author
Hi Ratter thanks for getting back to me. As far as my son is aware, this is the first infestation. It's a private company, they put a camera down and have identified two nests, one under the utility room and another larger one under the bathroom. Both have concrete (possibly suspended concrete) floors.
They have said that they may have to dig up the floors so that they can fully seal off any holes, otherwise, even if this lot of rats are killed, others may move back in.
Just wondering if it is usual to suggest such a drastic course of action, never dealt with anything like this before!