or perhaps
Ever since I became interested in citrus varieties over thirty years ago, I have been seeking out English specimens of the hardy citrus relative, Poncirus trifoliata - the Japanese Bitter Orange. At the same time, I have been keeping a watchful eye out for any sign of the hybrid varieties Citrange (Poncirus X Orange) or Citrumelo (Poncirus X Grapefruit). Apart from one claimed Citrange (which proved on examination to be a vigorous Poncirus) I have failed to find any.
However, in November 2010 I received an e-mail with photos of a small tree growing at the back of a house in the Wollaton area of Nottingham. With trifoliate leaves, and covered in fruit larger than those of Poncirus, this was clearly an example of the Citrus x Poncirus hybrid I that I knew must be able to grow in the UK. The owner believed it to be a Troyer Citrange, which he had planted about 22 years ago, and had been obtained from a nursery in Lincolnshire.
Interested in the history of this tree, and hoping to find out if there were any others, I contacted the now retired proprietor of this nursery which had specialised in unusual fruits and nuts. He replied:
I can’t remember the name you mention but I do remember calling in and delivering some plants to the Wollaton Park area of Nottingham many years ago when I was just starting the nursery. At this time I doubt I had any citranges and certainly wasn’t selling any citrus. However, I did have a lot of citrus seedlings raised from seeds a contact in the USA had sent to me and think this is where the connection comes from.
I  received the seeds from a Mr Zehnder who said the parent plant was a citrumelo, not a citrange. He didn’t give me the cultivar name of the parent plant so I initially called the young plants ‘Zehnder Seedlings’ although subsequently he told me that the true name was ‘Dunstan’.
Dunstan Citrumelo may be the best overall hybrid with 50 percent
trifoliate parentage. The tree is initially upright but becomes more
spreading as it matures, much like its parent the grapefruit. This
grapefruit hybrid produces 4-inch yellow fruit, which if sprinkled with sugar,
smell and taste like an ordinary grapefruit, harvested perhaps a bit too
early. Unfortunately, this cultivar is extremely hard to find, but worth
seeking out. Dunstan citrumelo appears to be extremely hardy.
A specimen in Mt. Olive, North Carolina endured the below-zero temperatures
of the 1980s that wiped out all other citrus in the area.
Flavor: Sour grapefruit, some off-flavors, fair to good quality. Use: Dessert.
Citrumelo or Citrange in Nottingham, England
'Hardy Citrus for the Southeast' by Tom McClendon is published by the USA Southeastern Palm Society. It says:

page created 14th November 2010
Citrumelo with fruit. Nov 2010
Citrumelo in snow. Dec 2010
Move your mouse over this picture to remove the snow!