Sillybilly I missed out the important 'wiring diagram':

use a test meter on ohms or a continuity tester to cheeck that:

pin 1 (plug) goes to pin 1 (socket)
pin 2 (plug) goes to pin 2
(socket).......................................
.................................................. )
pin 8 (plug) goes to pin 8 (socket)
This will give a 'straight through' extension.

Thanks very much Brian, most appreciated. I'm assuming the above wiring
is for the mic? it's good to see that it is all quite straightforward and
obviously can handle pretty long lengths too.

Yes that's the mic lead. Got the tape measure out and measured the rough
length of the extended mic lead, it's about 10 meters.

The 'head' on the 2800 only has 6 wires and I just 'snipped & extended' for
that using the 'extra (unused)' wires as earth/ground.