My A4 B8 2.0 TDI had a (slow) oil leak but this morning it has suddenly got much worse. Oil is spraying from the oil filter cap (no leaks when the engine is off), so must be a pressure issue. Does anyone have any experience of this type of issue and what resolved it? I'm thinking of trying to change the oil filter cap but I can't see any visible defects with it. Could it suggest an oil pressure issue/valve. My engine is CJCA apparently.

Yes you have excessive pressure in the crankcase that is blowing the oil out, and it is nothing to do with the filler cap. The usual cause is the EGR (exhaust gas return) valve being blocked. A less likely cause is more blow past at the piston rings than the EGR can cope with, though that would give other symptoms such as smoke and poor performance.

Thanks Cliff, I'll look at that tomorrow. Thanks very much for the quick response. 

In case anyone else has this issue, it was the oil filter cap. Replaced today with a new one and a Mann filter and no leaks. Must've been over tightened by last owner. Cheap fix in the end!