Every year I try and collect Poncirus trifoliata seeds for use as future rootstocks. But England seems to be on the edge of the climate region to produce viable seeds for this species.
I know of mature fruiting plants in about six locations within fairly easy reach. The plants all grow well.
So far this year, I have inspected fruits from the plant in my own garden - about 15miles west of London - and in the botanic garden of University of Oxford. In both locations the fruits this year are small, thick-skinned and seedless.

This follows a much cooler summer than recent years, with a particularly dry autumn.
About one year in every four I find seeds here are plentiful. Most years I find a few seeds, but so far none at all this year. There are still a few more plants to visit so I may yet be lucky!
Poncirus fruits in continental Europe are always stuffed full of seeds. The warmer summers seem to be needed for good seed production.

I believe pollination has to occur for seed to develop. Could the lack of seeds then be partly attributed to the decline of the honeybee in many areas?

To add to Mike's observation here is the photo (taken today) of P.T. fruit from continental Europe (my backyard):


The fruit from summer bloom has more seeds than the one from spring bloom.
Does it (well, small sample) suggests that warmth is indeed crucial in seed development?