The landing fee is an insurance issue. To avoid it you need to have a Crown Indemnity Clause written into the aircraft's insurance policy to the appropriate value for that weight of aircraft. The Clause must then appear on your insurance certificate. You then apply for a MOD Waiver certificate from some gadger at High Wycombe. (PM me for details) This certificate will then let you land at MOD airfields without having to pay a landing fee, but only at airfields that you specified when you applied for the certificate. It is not blanket cover to fly to any MOD airfield. You will also need security cleared as well this not a problem if you are employed by MOD just fill in your service/staff number on the form. If you are a member of a military flying club and the aircraft belongs to a club or is already kept at an air station the waiver certificate should already be squared away.
Hope this helps.
Regards,
Jucky