Sorry if this is common knowledge amongst the hard core and savvy shoppers but i think i should let it be known. I recently used a Sainsburys voucher (£4 off a £20 shop) in Tesco and they accepted it with no questions. This is not available in Asda as i have tried there. Also, the voucher number from Sainsburys do not work on line as far as im aware, but I could be wrong and im not sure if the tesco vouchers will work in Sainsburys.
Like i said, sorry if its common knowledge but its my first post and rather than promoting an item i thought i would promote something that can be used all year round (subject to obtaining the vouchers)