Your right to a refund, in respect of faulty goods, only exists up until the point where you've been deemed to have 'accepted' them. The length of the 'pre-acceptance' period isn't defined in law but it's unlikely that you could successfully argue that, after 9 weeks, you'd never formally 'accepted' the phone.
Once you accepted goods, which later turn out to be defective, the retailer is obliged to repair or replace any goods where a defect is due to an 'inherent fault' (e.g. poor quality components or poor manufacturing standards). That obligation lasts for 6 years. During the first 6 months of that period there's an automatic assumption (unless there is clear evidence to the contrary) that any defect is due to an inherent fault. (Thereafter, it's up to the purchaser to show that, based upon the balance of probabilities, the most likely cause of the defect is an inherent fault).
So the retailer is under an obligation to repair or replace the phone, but not to offer a refund. You, as the purchaser, have the right to choose whether you should accept a repair or a replacement but the retailer can reject your choice (and substitute the alternative option) if your choice is not economically viable. That means that you have the right to insist upon a replacement phone. The retailer might argue that providing a replacement would leave him out of pocket (thus invoking the rule about 'economic viability') but, unless his supplier has gone bust, he should be able to seek compensation from his supplier. Tell the retailer that you want a replacement. You shouldn't have to wait while he contacts Sony Ericson. He should provide one from his stock.
Chris