Jonny c

Hi all, I'm new here, I have a query that I'm hoping someone else has encountered. I recently bought an 11 plate a3 for doing motor way miles as I know they are economical, the trader sold me the car and the very next day I had oil in my collant, so I had a chat with him and he's willing to come and collect, but Ideally I want to repair it, ive changed the oil cooler, flushed the coolant system and done an oil change , all went well, then today again, oil in the coolant, it drives perfect , no over heating, no loss of power ( driven 200 miles with this issue), no gasket gunk on filler cap and my mechanic says i ts definitely not the head gasket, any ideas please before I start throwing money at it....return it ? or is there something simple I'm missing ? Internet says its a seal but dont know where to start. Thanks