Mid-March is really too early to put your citrus outside in the UK. However, I too was fooled by the beautiful warm weather. Worse, I had to go away for a few days and then at home the temperature fell to -4C (25F) one night. A lot of new growth was destroyed.
However, if you do not get significant frost, then growth will simply re-start when it gets warmer again.
Make sure your plants are not sitting in water and they can drain as freely as possible. Some years I have made a kind of plastic collar around the main stem, so that further rainwater runs off over the edge of the pots.

Here is a link that might be useful: HomeCitrusGrowers

citrange- just a note to tell you how much I am enjoying browsing around your site.
I was especially educated (and entertained) by your Parent Washington Navel page which I have linked below for others to stumble upon and be similarly engaged.

Here is a link that might be useful: Parent Washington Navel

cearbhaill -- Thanks for your notes about the website.
I'm afraid I don't really update it often enough these days as it takes quite a bit of time and effort. Also, I'm not travelling around so much to visit new 'citrus places'.
However, most of the information is still relevant and I do get bursts of enthusiasm - especially when I get appreciative comments like yours!

My citrus trees have been outside since March when we had warm weather. My Kumquat trees grow a lot of new leaves; I just put some good fertilizer and iorn in the soil to give them a good dose of "food". I also pinched off some of tip to shape the tree a little bit.

My Meyer lemon has new leaves growing and leave buds showing on the branch. I noticed about couple of weeks ago, and I am sure now, that there clust of flower buds are appearing. They do not grow in fast rate but grow steadyly. It seems to me, they are well adapted to the cold weather here and know the spring/summer is coming reagrdless the low temp. at night.

I leave my citrus outside all year. Last year my variegated lemon started putting out new leaves in the late fall right before it got cold. When it got cold, the leaves were about 1-2 cm in length. They remained that way for the entire winter until they started growing again when it warmed the following spring. The leaves looked completely normal when they were mature.

Thanks everyone. I can see lots of new growth ready and waiting .... I guess it will just keep waiting until some decent weather (pleeeeeeze!!) comes along.

Consider me reassured by your comments!