The article and re
transponders. The conclusion:

I believe it's ok for us to fly in class E airspace. See section
4.7 (ii) - gives us access to E space. 3. 1(b) exempts us from VHF radio. I
believe the transponder exemption is described in the Aeronautical Information
Pack (1.5) section 6.1.2.

The instrument CASA 202/98, I think, which specifies transponders for class E
airspace only applies to aircraft with engine driver electrical systems.