Ok, this will sound revolting but I tell it so you know how BAD a situation it was.
I suddenly became aware one humid, very hot summer that my flat had become infested with fleas to such a degree that EVEN THE CAT WOULDN'T WALK ON THE CARPET (that's how I noticed it at first, the cat was jumping from high surface to high surface to get anywhere and watching the carpet with wide-eyed horrified / paranoid intent..) I looked down and literally every step I took, about 10 to 20 baby fleas were on my legs. Eugh!! Anyway, I got the smoke bombs (Thank God for Amazon's always reliable super express delivery), set one off in the living room, one in the hallway and one in my bedroom. Two hours later (after sitting in the safety of the garden with my cat meanwhile),nothing: no sign of a single living being, everything dead, finito. Cue earie silence as the breeze from the now opened windows moved the lifeless shells of the poor hiding spiders I'd not managed to evacuate beforehand...
They are smoke bombs (ensuring the poison permeates into everything including blankets etc), so it will smell a bit smokey after but once you open all the windows, it disappears pretty quickly and God was it worth it. I've bought my next supply just in case anything like it ever happens again. You don't need to call in £100+ professionals with this stuff I tell ya!
(NB: I just got the smoke bombs, the trigger bottle spray didn't come with it so I can't say how effective that part is)