Kitchen tap cartridge- help needed

Hello all, 

I have a 2004 Ace Jubilee. The kitchen tap micro switch has failed, and I would like to change the cartridge at the same time. 
live managed to remove the lever with a small grub screw, the the top ring has unscrewed but now I’m stuck.

i can see a brass nut (29mm by my own measurements) but is is recessed in the body, without clearance for a socket.  The outer sleeve will turn on the thread around 3 revolutions but I’m absolutely stuck how to remove either the cartridge or outer sleeve.

i am at the point of just buying a new tap but I’d rather fix than replace.

photos below of the tap, showing lack of clearance for socket as described. 
Any help and advice would be great, I’ve searched all over the web but can’t find this particular problem shown anywhere.

John