Hi Barney,
I am not sure about the ability to obtain cheap updates, but updates can be obtained directly from Bendix King at Chichester, tel. no. 01243 783763, at £125 per update, which I believe are issued monthly, although an annual one may suffice for most users.
Alternatively, I imagine both Transair and/or Harry Mendelssohn, will probably be able to assist in this regard.
I hope this might be of some help.
Paul A.