Sign up for tesco dvd free trial and get £4 quidco tracked to your account.
I can't put a link on for the quidco people, but click the link if you want the free trial without quidco.
In the URL of th link (which I copied and pasted in after visiting it through quidco) the promotion code is TDR2DT so that may be all you need. Not sure how it works sorry.
EDIT: Get two months of free dvd rental with this code as well: TDRRAFCT9 (found by jetsetwilly) , though i'm not sure if it works with quidco this way.