I have been carrying out pest control for years now (flying insects, lavae, beetles, not rats).
For heavy infestation Ashbys Insecticide is very effective
For anything up to an infestation I could not recommend M Power enough.
It really does kill most problems in a few minutes & the over-spray that you'll always leave behind is good for about 14 days.
My chickens suffered red-mite last year & I used to use a DEFRA approved product which stank & had limited effect. I used M Power (cold @ 1:200) & it took just 4 minutes for the poor chickens to relax & the mites just fell off!
For fleas I use it in a small fogging machine & to date have never had a call back!