I am not aware of any commonly grown citrus which requires another tree for fertilization. Having made that statement, I'm sure someone will tell me some obscure citrus does require another tree to grow fruit. But note I said commonly grown. When my citrus bloom during the winter, I use a bird feather to pollinate the blooms since I don't have bees in the greenhouse.

You can happily grow a single citrus tree and it will produce fruit. Most varieties are self-fertile, and many do not need any pollination at all. They are botanically called 'parthenocarpic'. Often, cross-pollination will simply increase the seed count in the fruit.
I enjoyed looking at your garden web-site. The last plant photo (right front garden) is of an Echium. A biennial, and hardy only in the mildest parts of the UK. I am growing one in a pot - it was given to me by the gardener in Preston Park, Brighton where they seed freely from their tall, blue flower spikes.

Hi,

My citrus is a Citrus citrandarin changsha. It is currently in flower so I will wait and see if it sets fruit unless anyone can advise specifically on this one.

Yep, the echiums flowered this year and one was a bright pink one. I think it might be Echium Wildpretii. They are completely hardy in the bit where I live and they self seed everywhere. Now the seeds (millions of them) are ready, but it is too wet to harvest them.

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Rubble, what a beautiful plant..it's huge and those flowers are stunning..How long did it take to grow the size it is? Toni

Echiums are biennials, so 2 years, then they self seed and take the whole garden over!!!

Rubble, so they come back every 2 yrs? Will it continue living on? It's truly gorgeous..Toni

Many citrus trees are indeed self-fertile, or don't need fertilization. Others do need fertilization and won't set fruit without being pollinated by an unrelated pollen donor.
Clementine Mandarin as well as some of it's progeny need cross fertilization by a compatible donor. Satsuma mandarin as well as Washington navel oranges set fruit without pollination.

I've got a Lemon Meyer which flowered but sadly never bared fruited this years. After searching the net I found that it helps greatly to have another citrus in the same area and from what I've read it doesn't seem to matter what type of citrus it is as long as it's a ctrus.

Here is a link that might be useful: Manuli Packaging