Oki, firstly it's known as Skirmish J20, not Leicester, cos its not taht close to Leicester actually..... anyway, no postcode that i know of but is pretty easy to find....here's all you need to know
Head towards Junction 20 of the M1, then follow the signs to Lutterworth, when you get to an island with the jet airplane in the middle you should follow the brown signs for Stanford Hall.
This will take you back over the motorway. The site entrance is then located on the right hand side after about 1 mile.
Taa daa