Firstly, small rant, it annoys the sh!t out of me when a main dealer condescendingly suggests, often without even looking, that its not worth your money and their effort to address a fault that you've asked them to investigate. Its lazy on the part of the dealer too.

Not sure about the layout on the 520d engine, but its not uncommon for petrol 6cyl BMW's to weep a little oil from the rocker cover gasket and it drops straight down on to the exhaust manifold. From your description it sounds like something of that kind may be whats happening. Its not a wallet breaker.

John Roche is in Gort, so, not far over the county line from you. He knows his BMW's. I've been going to him since 2004 and can recommend him heartily. He may want the car for a couple of days to diagnose, order parts and repair if that's what you decide you want to do. PM me if you want his contact details (and say hello to our 316i if you're there in the next few days!