Got it sorted, and went to the trouble of putting the belts nearest the engine. My tips are:
Remove the fan coupling
Liberally spray down the adjuster with WD-40
Back it off as far as it goes. This is painful and the generator will hit the coil mount when there is still room to go on the adjuster. Removing the coil may be an option, but I didn't try it.
Wind in the other nut on the adjuster in, to hold the generator at the slackest position. Finger tight will do.
Cut the old belts off. You'll never get them in one piece.
Lube up the belts and pulleys with plenty of liquid soap.
Put the belts on the bottom pulley first (there is a bolt or something under the bottom, so not much clearance. Prize it in with your fingers).
Then go round the generator.
Finally the water pump. A lever will help get it over, as will someone turning the crank handle.
Pry the belt over on the crank pulley, while someone turns it.
Repeat on the water pump.
Feed a shim between the generator pulley and the belt from underneath and when it's lifted clear of the belt, pry it over to the next row. The best shim I found was the flattened cardboard tube from the centre of a roll of duct tape, fed in lengthwise. It was strong enough to hold the belt clear of the pulley, pliable enough to wrap around the pulley, long enough to cover most of the contact patch and grippy enough to get pulled under the belts, even while covered with soap.
Repeat until both belts are in the correct positions.
Refit the coupling to the fan.
Back the outer nut on the adjuster right off.
Tension the belt. Turn the engine over a few times with the crank handle while doing it.
Run the engine, while gently hosing all the soap out (make sure an access plate has been removed).
Recheck tension.
Tighten outer adjuster nut to lock it in place.
Drink beer.
Cheers,
Terry