>>>I said that if a product is incorrectly priced in a shop, the shop has to sell it to me at the marked price
That's hardly a good argument to use, as it's completely untrue. As long as they avoid specific forms of discrimination - such as on the grounds of race, gender, etc - a retailer is free to charge every customer a totally different price (or to refuse to sell the item to them at all) irrespective of any price marking.
If you were specifically told BEFORE the original repair was done that the service offered included a warranty lasting at least 4 months, then that statement formed part of the contract under which you agreed to to have the repair done. Otherwise any warranty referred to is simply a 'gift' from the repairer, who is entitled to modify the terms of that gift (or to withdraw it altogether).
That, of course, is entirely separate to your statutory right to receive an adequate service from the repairer. If you can show that the most recent problem has come about because the repair 'wasn't up to standard', then you're entitled to a free remedy now. However if the current fault has come about through, say, poor manufacturing standards, the repairer has no further obligation to you.
When your phone first went wrong you should have demanded that the RETAILER fix it free of charge, on the grounds that there was probably an inherent fault (such as a poor quality component) when you bought the phone. That right exists for 6 years from purchase and is totally separate to any manufacturer's warranty.
Chris