A pollination mystery?

Some years ago I grafted a multiple variety pear tree for my parents, the idea being that three varieties on a single tree would cross pollinate efficiently and give a succession of fruit. The varieties were Bristol Cross (as South Wales is very wet compared to Oxford, so a particularly scab-resistant variety seemed a good idea); Dr Jules Guyot and Concorde. The first two are in the same pollination group, and Concorde is slightly later; both Concorde and Dr Jules are reputed to be partially self-fertile, whilst Bristol is self sterile.

This tree has been in about 8 years, but has never produced any fruit. This year it has set a small number of Dr Jules only. For once I was at home to observe flowering, and all varieties did flower, though some of the Concorde blossom did appear to be later than the other varieties. Even so, the Dr Jules should have provided pollen for the Bristol Cross (it obviously flowered this year, as it has set fruit).  Frost shouldn't be a problem as the climate is much milder than here. 

I think the problem with this tree must be some sort of incompatibility between these varieties. I did quite a lot of research before grafting the tree, so I'm quite annoyed the combination has worked out so badly.

The only solution will be to add an appropriate pollinator, either as graftwood or by squeezing in another small tree. It seems a shame as the tree is growing well and healthily and the three varieties seem very well matched in vigour, which can often be a problem on 'family trees'.