I'll second that CK, these car supermarkets are big on their incremental sales (GAP, PPI, Ext warranty et al), these products offer a commission of a minimum of £50 per product to the salesperson, hence the hard sell.
Used car supermarkets have good and bad reputations based on what type of deal you get. If you can get a car with some manufacturers warranty left at a reasonable price and at a good APR then yes, Motornation are ok.
If you are weak with the salesman and get slapped into a car for too dear, at a high rate of interest and accept that the extended warranty is better than the manufacturers one, then after a few months when people find out what you've done, Motornation are the worst in the world.
A used car dealer will try to get away with whatever it takes to make profit, it's up to the buyer to make sure you don't get had over.
JJ