Question: is it mandatory? and if not do they charge you an insurance premium when you land?
Any advice most welcome.
Not mandatory - if you don't have it they will charge you a supplement every time you land. Used to be GBP 8.50. don't know if that's changed.
Might be worth having a chat to your insurers - they may add it for no extra charge, or at least a nominal admin fee.
(I've operated into Lyneham - all Ops wanted to do was see an insurance certificate with the words "Crown indemnity" and "GBP 7,500,000" clearly visible).