Chris says he is aware of the Service book method but has trialled it many times and it doesn't make a difference to the tension either way.
I seem to have opened up a little can of worms there! Thanks to all whose contributed, ill get a pair bought and changed. I didn't intend to put the bike on the road for a while (as its a bit awkward to get it out of the dining room!!!) but the insurance renewal came round and it wasn't much dearer with the 851 added to the policy. I've always wanted to have / ride one after all! Umm, as a ps, where seems best for belts? Moto- Rapido?