"£5 from a market computer stall".....Ah, that kind of explains it
Loads of fakes and poor quality stuff floating about out there.
Check out Tesco, I bought 2 16GB Sandisk flash drives (stocking fillers for my nephews) for £4 each (5 year guarantee), in their clearance sale.
sold out of those now, but, just as an example you can get a 16GB Kingston USB 2.0 flash drive for £5
http://www.tesco.com/direct/kingston-16gb-data-traveler-usb-card/526-1525.prd?pageLevel=&skuId=526-1525If you have USB 3.0 on you PC/ Laptop (or will do in the near future) then go that, the price per GB drops on larger drives, so also look at 32GB and 64GB too.
I've got a few flash drives, Kingston, Verbatim, Integral and Sandisk are all pretty reliable brands to look for, from reputable sellers