Cassette toilet service door lock

Hullo,

Maybe a bit of an odd one. 

I want to repair the lock on the toilet service door of my Symphony. However, I need a box spanner to reach the backing nut. As I do not have a set of deep reach sockets or box spanners I need to know the size of the nut so that I can get the right size.

Sadly my local accessory shop does not have a spare lock assembly otherwise I could just pop in and measure it. 

Any suggestions please?

Many thanks in anticipation.

B.rgds,
RMG