If the warrenty is from 'Digital Satellite Warranty Cover Ltd' then you might take a look at this thread on the Sky user forum.
http://tinyurl.com/2poay9
This company and/or similar are always phoning Sky subs pretending to be Sky to sell an extended warrenty. Whether or not they are worth the paper they are printed on I have no idea although I would be surprised to find they had any value.
Having been involved in appliance servicing for much of my working life I am of the opinion that extended warrenties, generally, are a waste of time and money. In keeping with most schemes where you pay up front your service call will be treated with a distinct lack of urgency as you are of course a captive audience. When and if your service provider can spare an engineer from work that has not been paid up front for you might get a call. If the Engineer doesn't write your appliance off after a cursory inspection then be prepared for a long wait while they procure spares. In stock spares being kept for non paid up front customers.