Rosie, there are different sizes in Halfords, go for the 3.5mm internal diameter stuff.
If you've got a TDI then you need to remove the intercooler, if you look at the download section there are a couple in there about other tasks (think one might be checking the glowplugs) that include details about intercooler removal - its only 4 small bolts & 3 or 4 bigger bolts,plus a couple of clips on the pipes.