Mark.
As Gareth said, your choices are to sell the car in its present condition or to investigate repair by removing the head. At one time there were companys who would rebuild blocks, heads and even whole engines, but I get the impression they are now rare. That is something you would need to research. A secondhand head from a breaker's yard is a possibility if it comes with a cast iron guarantee.
I would not worry about the bore scope report of a little oil on the pistons, that is to be expected as some of the valves are not closing, as long as no damage was visible on the piston crowns. Damaged pistons would add greatly to the cost.
I still feel the garage that changed the belt bears some responsibilty providing it did the job properly, including any recommended change of water pump and belt tensioner etc. If short cuts were taken to reduce the cost by not carrying out all those tasks then its liability is reduced or eliminated.