She's very beautiful. I'm such a sucker for anything pink! Nice photo.

A beautiful flower, I think one of the violacea group hybrids. which are all P. caerulea x P. racemosa. Pink Princess is a made up name that is not correct. See link below and http://www.passionflow.co.uk/eg111.htm for examples of the violacea group.
Myles

Here is a link that might be useful: Passiflora violacea

I suspected a made-up or mis name.. as it does look quite close to violacea.. Without really enough difference(s) to warrant it's own name.

I wonder whay caerulea is used in so many crosses? Is it because (like I) it's what the grower had around, or is there 'something special' it contributes?

I think caerulea would be a good choice for hybridizing because it is so cold hardy and vigorus. Most pf are tropical and tender except for zone 9 and above. Many pf are small stemmed and delicate. Genes from caerulea produce pf that can survive better in Europe and North America.