Sussed it. Feel like a bit of a pleb for not noticing it the first time i had the rocker cover off, but its a proper 'jeeezus' moment. I decided to check all the torques on the bolts to ensure nothing had come loose. That was when i noticed that one of the four bolts holding one of the injector rocker rails on wasnt there! Then panic set in - if it wasnt there.. where the feck was it. Out of sheer and blinding luck, i felt right down into all the oil ways and nooks in the head and found it, and managed to get it out with a circular magnet attached to some strimmer wire (didnt want more metal flying about in the engine). One lucky escape. Thing is, looking under the head of the bolt i found its flush. The bolt left in the engine is sat about 2mm into the hole where it sits. So i started fishing again and found nothing more after an hour. So i took the oil filter off, and again came up with nothing. I intend to get the other three bolts off and see if the head of the bolt matches up to the rest of it. If not, ive got about 2mm of metal kicking about in the engine somewhere and its sump off time soon as it stops raining. Im aware these are stretch bolts and have to be replaced each time, so im *hoping* that it does match up once i get to it. Four new bolts on order from VW cost