I'm not so familiar with the TD5 engine, but at a guess it is an oil breather. On the TDi engines there is a similar pipe from the oil breather tank to the air intake beyond the filter to feed any oil vapour back into the engine if you have crankcase pressure.

I'd echo Pete-B I bought a similar spec item off amazon This is the review I wrote about 3 weeks ago 5.0 out of 5 starsOutstanding! By Conedxf on 4 Jun. 2017 Colour: DJS50 Black/Yellow Verified Purchase I admit I was sceptical about the claims made for this device, however I am happy to say...