I haven't got a code but in my local Tesco at the self-service checkouts they have a load of leaflets trying to get people to use tesco direct (Online tesco delivery thing)

You get the voucher, register online and enter the code on the coupon. (Codes can only be used once, hence I haven't posted any here.) You have to spend at least £50 and it has to be on groceries but I think a £50 shop entitles you to free delivery anyway and if you spend this much on shopping anyway you're laughing. If not and you can afford to/have space, you could always use it to stock up on things like tins, pasta, shampoo etc that doesn't go off.

The leaflet says "£5 off your FIRST shop" but I can't see anything in the small print that says you can't use it with an existing account, however I would assume it is one per person (account.)

If you can't find the vouchers at your local Tesco you could try asking at the customer service desk. I found mine by the self-service checkouts.