Tried the belt slip today, the noise is still there. It obviously did something, as there was a little bit of noise coming from the belt that stopped immediately after spraying, but 10 mins later, still the tssh tssh noise. Will leave it for the rest of the weekend now and try it on Monday, but doesn't seem to have worked so far!
Any other ideas? As it's not causing any real issue, I'll probably just live with it!
Can't remember the exact dates, but the later mk 2.5 models had a coated DPF. The date was definitely in 2009, as I just missed it (mine's a December 2008 model).
Anyway - @exponential - I'd look at the other issues. Based on your driving, and my experience (along with someone else I know who has the 2006 model), I do not think you should even notice your DPF is there.