Well we can certainly see microlights on our screens. It's a good job since we talk to quite a few out of Hewish, Weston Zoyland etc
The return depends on many things, mainly your altitude/distance. In theory we should be able to see you at 1000' out to ten miles, 2000' out to 20 miles and so on, but this also depends on the atmospheric conditions eg if you are flying into a decent headwind or there is a bit of WX clutter, the return may be filtered out.
Most of the time though, you will show up. Our particular problem at Bristol is that lower aircraft are masked by the Mendip ridge, however, if you were to fly down the coast from say Slimbridge in Gloucs to Hinckley Point, we'd prolly see you all the way even at 1000', but then again that won't help you much when you're flying in Cambridgeshire, will it?