Well, my plan has backfired.
I followed the trail of cigarette butts (like Daisy suggested in her comment last night) and it didn't lead to cheeky squirrel's den at all, it lead to braer fox's fox hole. And, there was a big pile of butts right at the entrance.
This means the squirrels haven't been smoking the cigarettes (which sort of explains why they're as fast as ever). It's been that cunning fox the whole time!
You know braer fox is my mortal enemy? He tries to sneak into the house when the garden door is open to steal food and once he chased me across the garden and into the cat flap at such high speed, I broke the cat flap. When the humans found me my tail was so fluffed up it was bigger than I was.
I guess I am sort of relieved. I didn't really want to make the squirrels sick. FAZ