Quick update had the rat man round he said he thought it was rats, left a load of poisen around the place. Said that hopefully they will die outside, likely they will die inside in which case it can be a pain to find them but not alot he can d about that! (smell terrible apparently for 5 days to 5 weeks!) Fingers crossed it doesnt happen.
Good idea about iron wool, he said that he thiks they maybe coming up broken pipe around bottom of stack, so im wondering if i seal all round it with your method where it joins the floor that may sort it!
Thanks all!
Sods law they will die in a cavity you cant get to. Hope they don't stink too much lol