John
The CAA / Europe don't require enhanced mode S for helicopters, only elementary surveillance which as I said is mode S + flight ID.
Enhanced mode S requires loads of aircraft parameters to be input to the transponder such as bank angle, speed, altitude presets etc so you have to have a substantial mod to able to fit it - hence even the CAA realise its not practical for a small (or even large) helicopter.
Kevin, Flight ID makes no difference to the CAA's capability to invoice you, they already have your 24 bit unique address from the basic mode S which is registered with them (look in G-info).
HC