When I change down into 2nd gear, it grinds. Changing from 1st to 2nd is a bit clunky, but it doesn't grind like going from 3rd to 2nd. It's a VW van - LT 35 TDI LWB. Does anyone have an idea what it could be and how to fix it?
That sounds like a worn synchromesh on second. To test it, try double-declutching when you change to second - i.e. press the clutch, move the gear lever from third to neutral, release the clutch, press it again, move neutral to second, release. This slows down the gearchange and gives everything in the gearbox time to adjust speed (and was how everyone had to change gear before synchromesh was invented!).
Unfortunately, fixing it involves rebuilding the gearbox, so it would probably work out cheaper to get a new or refurbished gearbox fitted. Or, if it's an older van, get good at double-declutching and learn to live with it!
The first thing I would check is that the oil in the gearbox is at an appropriate level and I'd consider doing a gearbox oil change. If that doesn't cure it, I'm afraid it's a case of strip the gearbox so it may be a case of changing the oil and seeing if you can manage the issue by slowing down your changes and maybe double declutching until it gets to the point where it's so bad you need to rebuild or replace the gearbox.