I spoke to the CAA today (after much confusion: apparently 'administrators' don't know what the ANO is!) - from the 25th any UK registered aircraft that is not subject to Annex II (which are exempt) will require an ELT if it is 6 seats and greater, or if it is less than six seats an ELT or a PLB. The person I spoke to suggested another exemption would not be made...
I hope this clears it up a bit!