Firstly thanks to Jim, Malcolm and rest of the forum, once again for helping me sort out my car.. it's really great.
So I put the 6PK1725 auxiliary belt on my´96 1.9TD Xantia. This ´tensed´much better and the belt is running smoothly without any noises... in fact it's as quiet as it's ever been.
There is room for a shorter belt, as the manual tensioner is still near its upper limit, but as I'm wary of overtightening and it runs smoothly, if it aint broke.... etc.
Therefore my very minor contribution to this forum from this experience is as follows:
Lessons learnt (assuming my car is not atypical)
A. Citroen dont always get it right! They assured me 6PK1736 for this setup
B. Car parts catalogues tell porkies too. They also gave me a 6PK1736.
C. Ideal belt sizes between 1710 and 1725. Although sizes smaller than 1720 I reckon will be very hard to put on.
Cheers and wish me luck for my annual pilgrimmage to Citroen's Mother country.....
Larry