On a good day with no weather clutter and a radar that's just been 'tweaked' by the tels guys we can see you at similar range/distances mentioned by standard noise. If you fly at a tangent to the radar head you'll disappear,so if you fly between EGHH and EGHI fom say stoney cross to the lymington araea or vice versa you won't show up.If there's weather showing on the radar you'll be lost in the clutter and if we use filters to improve the picture you may disappear as well. Microlight pilots who call for a FIS and give position reports sometimes don't match with any discernible return.Given the amount of pax jets etc now flying around the solent/new forest area i'd advise you always to call up (obviously you must within class d anyway but there are areas between the two airports that are outside controlled airspace).
All this is primary radar, get a transponder and you'll be seen unless you are very low.