The reason for the failure could be failure of the engine crankshaft oil seal, the gearbox input shaft oil seal, the clutch slave cylinder, or the driving of the car, either by you or the various people who drove it before you. Since you weren't there when the engine and transmission were separated you can't prove the seals or slave cylinder. But if you take this to Small Claims you could allege that the failure could have been present or developing before you bought the car or could have been due to the seals or the slave cylinder, so you might get a ruling in your favour. Tell the supplying dealer dealer that if he doesn't repair the car FoC you will take the matter to Small Claims. See: www.honestjohn.co.uk/faq/consumer-rights/