Assuming you are talking standard IFR ILS traffic - not SRAs;
Biggin and City Airport both have an ATM (identical I think?)
Biggin need 10mile checks (to help with their circuit)
City only need them when ATM is not working. (no circuit traffic of course!)
Biggin also get a 20 mile check as well when they have the crossrunway in use.
It seems a bit of overkill, but then you have a busy VFR airfield with crossing traffic/runways do you not??