Hi Top Cat,
Normally, if a Chinese Elm is going to lose its leaves, it will do so in late autumn to winter time, as is the case for deciduous trees. If your's has lost its leaves subsequent to the frost of a few weeks back, that might suggest that the roots suffered some damage as a result.
It might be an idea to post a picture of the tree and provide a little more information about it. I'm in Northwest Kent, so not a million miles away from you. Both of my Chinese Elms were kept outside over winter, although they were in a sheltered position for the most part, and are putting out lots of new growth.
The first thing you need to do, is scratch off a tiny bit of the bark on the branches and at the base of the trunk. If you see a green cambium layer, your tree is alive, although probably a bit worse for wear due to the frost.
If this is the case,
DO NOT fertilize the tree, as the chances are that it suffered root damage and by feeding it, you will "burn" new shoots with the chemical fertilizer. Just leave it be, water when necessary and it should recover.
If you do not see a green layer under the bark, then I'm afraid you have yourself a dead tree my friend.
Fingers crossed for you!!
All the best
Aaron