Hoho jigjag (I quite like that phrase)
While I have sympathy with your non-communication with the M & S salesgirl, I feel it may have been a bit much to flash your knowledge of Insecta at her.
Nevertheless, my dentist recently refilled a tooth much filled before, saying "I think we should do this (like the "we"!) even though I'm worried it may open a can of worms." Some weeks later, after she had done the operation, I asked if any annelids had appeared. Luckily not.
I am going to ask a silly question. You must have bought jeans (or at least, trousers, I hope) before. Why not look at the old labels, possibly make a mental adjustment and try on a few pairs in the region of.....???
I ate a fried grasshopper in China. Not Diptera, but similar tasting I imagine.
The last twice I have left M &S without buying anything. Or, to be more precise, the first time I left with 2 jumpers made from recycled polyester. The next time I had taken them back. Do not go there (recycled polyester), you have been warned.