We had a similar problem.
The nation agreement states that groups of staff can choose to use existing arrangements (for up to 4 years while the AFC arrangements are reviewed) or switch to the AFC conditions from 01/10/2004.
The Final national Agreement and the Terms and Conditions of Service Handbook are available here http://www.nhsemployers.org/pay-conditions/agenda-for-change.cfm
Read section 2 of each.
Also take a look at the various union sites and ask your union rep.
In our case, staff side made a local agreement to retain all current arrangements without checking if anyone would be better off on the AFC conditions or indeed asking those doing on-call. Once we finally got them to admit it we had to put a grievance before the Trust would allow us to use the AFC rules.