The removal of a seal might invalidate a manufacturer's warranty but (unless a retailer goes bust) such warranties are completely pointless anyway. Any items you purchase from a retailer must be 'of satisfactory quality' and free of 'inherent faults'. If something goes wrong, the retailer must put it right (which is why you don't need a manufacturer's warranty). The removal (or absence) of a seal cannot remove your statutory rights.
In most circumstances, when a claim is made against a retailer, the trader can choose whether to repair the product or provide a replacement. In your case though, you discovered the fault soon after purchase and, therefore, you never legally 'accepted' the goods. This means that the retailer has to provide a full refund. (You can choose to accept a replacement product instead but you are not obliged to do so).
The Department of Trade & Industry website provides more information:
http://www.dti.gov.uk/ccp/topics1/facts/salegoodsact.htm
If quoting the relevant legislation doesn't get you anywhere, you should contact the Trading Standards department for the area where the store is located. (If this is outside of a metropolitan area it will be part of the county council rather than the district council).
You can also contact the government's new Consumer Direct service:
http://www.consumerdirect.gov.uk/
A quicker way to sort out your type of problem, however, is often to involve the local press or BBC radio station. (It's amazing how quickly some problems get solved when a store manager is worried about having to explain some bad publicity to his bosses!).
Hoping this helps,
Chris