Thanks, Meldrew's mate - I CAN believe it !!
Could I pursue this a little further with you.....
The ep2000 programmer has a terminal 'block' labelled
A,B,C,D,N,L,1,2,3,4,5
and has 4 cables wired into it (all have earth cores connected)
Cable 0 is the one that goes on to the boiler...