When I first started collecting and propagating citrus almost thirty years ago, I found it impossible to obtain viable Poncirus trifoliata seeds in the UK. This was why I first started seeking out fruiting plants, and later started my website listing mature specimens.
Over the years, I have found quite a few sites myself and been told about several others.
Today I was amazed to discover a veritable hot-spot of Poncirus plants right at the spiritual centre of London - in the churchyard of St. Paul's Cathedral!
There are plants on the North, East and South sides of the cathedral, although the northern plant is probably always in shade and was not fruiting. The others are fine, old specimens covered in orange fruits.

I am currently constructing a web-page about the plants - more photos will be coming to
http://www.homecitrusgrowers.co.uk/wheretoseePT/stpaulscathedral.html
Mike/Citrange

Brilliant pictures! Whenever I'm down I find a quick reviewing of your site cheers me up no end. Keep up the good work.

Good find mike. Those are some huge trees.

What was the fruit like?