I suspect you just have a bit of slack on the belt in some parts of the belt run, there's a bit of a knack to getting belts on. If you are having trouble keeping the belt really taught between pulleys try using cable ties to hold the belt on each pulley. Assuming it's the correct belt it really well go on, make sure tensioner is completely backed off.
I think my problem is at the bottom, crank to aux pulley, where it is largely hidden behind the v belt pulleys. I will try again when i'm feeling competent!
To let you know it was all ok in the end. replaced and running. The tensioner was pushed to what felt like a hard stop but with some careful leverage against the aux pulley, it went far enough to get the belt on.
In the end I used this method, as opposed to finishing on the intake cam:
Fully slacken off tensioner (as above)
Get belt around crank pulley and on teeth properly (hard to see with the v belt pulleys in the way)
Pull tight across right hand side of aux pulley
Pull tight across intake cam pulley: if there is slack in the run to the aux, take up by slightly adjusting cam anticlockwise. hold in place
Put around exhaust cam pulley, as above take up any slack by turning pulley anti clockwise slightly. now hold belt in place here at around 10 o clock
Now working from the left hand side of the crank, make sure there is no slack to the left of the aux pulley (only a few teeth touch here, and access is hard with the v belt pulleys in the way)
Most of the slack is either now side of the tensioner, and the belt will just about go over it.
Release tensioner, turn over, check alignment. repeat this step until belt is properly tensioned.
To double check turn it over by hand to check the belt isn't wandering.
This is easier with a helper to hold the belt on the exhaust cam while you fit it over the tensioner, also helps to have flexible hands as you are working between the inner wing and tensioner. As far as fiddling with the cams, i aimed to venture no more than half a tooth from where they started, though it helps in insuring you get the belt on properly, then take up the slack, rather than trying to put it on fully tight.
Painted marks are useful to supplement the official marks, and pictures are great for checking that marks are lined up as before.
Probably not the right method: please correct me, but it hasn't broken yet!
I used a gates 5022 belt and a 3205A-2RS1TN9/MT33 SKF Double Row Angular Contact Bearing 25x52x20.6mm for the tensioner bearing
Cheers