Are you sure that you didn't, at one time, have an account with Abbey National (which later became just 'Abbey')? If so, you'll probably still be on Santander's mailing list. Alternatively it could just be a standard piece of marketing, based upon information shared by another company (such as the people who phone you up and ask if you'd help them with a 'survey')
Scams by post are fairly rare (largely because of the cost of mass mailings). If the envelope bore a stamp you should probably be suspicious. If it was franked then it's probably OK. If it was sent by one of the bulk posting services (such as TNT Mail) it's probably also OK.
As Chuck suggests, simply googling the phone number might help.