Can't get new auxiliary driver belt on CDi

[zaph]

Replaced the alternator and found a chunk missing from old belt. Bought replacement belt from discount rover supplies, along with a new idler.

The problem is the belt is not even close to going on. It is the correct part number, but the amount of movement I can get with a ground down aerospace spanner on the tensoner is not enough to get it over any of the pulleys. Which ever pulley I try I have only enough slack to get round the top half of the pulley at most.

The spanner is fouling on the subframe, so there maybe more movement in the tensioner. Any ideas?