FAULT SOLVED 
Just thought I would update with a great result!
I had a week left on the 30 day warranty I got with the dealer (Autoworld at Chesterfield) so called back there before booking it into an indy to get sorted. I went in armed with the assessment the Indy had given me (likely the cam chain, tensioner and adjuster need changing), printed a schematic drawing of the engine showing the three parts and highlighted them and wrote a detailed description of the fault.
Dealer asked me to drop the car in so they could have a listen and to my utter amazement said 'yep, dont think it should sound like that, we'll get you booked in an sort it'. I have to say, I absolutely expected 'they all sound like that mate... tata'.
Dropped it in to them on Wednesday, got a call the same afternoon to say they had ordered all three parts, then got a call this afternoon to say it was all done. Have to say, even at this point I was not expecting it to be sorted although I had a little faith due to them being a franchised Seat dealer and workshop.
Picked it up and OH MY GOD! Its like a new car, absolutely silent on tickover, feels tighter and sounds wonderful... the TDi effect is completely gone, cant express how good it feelt pulling into the petrol station and no one shouting that I was putting petrol in a diesel lol!!
They have indeed replaced all three parts just to be sure, Cam chain £90 Gasket £30, Tensioner £80, Cylinder Head Gasket £40 Cam chain Adjuster £300 and about £350 of labour so about £850 in all, all covered under the basic warranty.
I was told if it had been left there was enough slack in the chain that it would have evntualy either snapped or jumped taking the engine with it.So, massive respect to Autoworld and the helpful staff there... no quibbling or arguing, just sorted it quickly and got me back on the road, and if your GTi sounds like a TDi, its and expensive fix but well worth it.
