You should have a vat receipt to claim back input tax, as far as I've been led to believe. I always thought large supermarkets did give you one to be honest, as long as the goods are subject to vat.
Most big stores give a simplified VAT receipt. http://www.hmrc.gov.uk/vat/managing/charging/vat-invoices.htm#6
If I remember right Tesco put a * next to items which have VAT on them at standard rate.
Kris
Yeah and asda put a V next to theirs and if I remember rightly adds up the total of vatable items so you have a net figure to input, or it could be another supermarket, I shop around
Although diona is totally right, in the real world I've never known anyone to be pulled up by the taxman for small receipts which don't show the vat rate as long as you can identify what the items are and what the vat would be on them. I've had 2 clients visited by hmrc and neither took issue with this.