Potters Bar to Lodon Terminals

[cjp]

Monthly season as above - route any permitted, no Maltese Cross.
Monday to Friday it gets used either on the line up towards Old Street and Moorgate or into Kings Cross. No rail weekend service into Old Street or Moorgate so may it be used on the tube from Kings Cross to the Other London Terminals?
I have looked here http://www.railforums.co.uk/showthread.php?t=84351&referrerid=13671
and here http://www.railforums.co.uk/showpost.php?p=655286&postcount=7

If the answer is yes I envisage issues with LT staff but perhaps I am being unfair to them?

Map WA seems to be the one.

 

[maniacmartin]

Due to a rather peculiar easement-style rule that's existed for many years, all National Rail tickets (except maybe APs?) that have validity between Finsbury Park and London Terminals are valid into Kings Cross, then by London Underground to #Highbury & Islington, Old St or Moorgate only. Break of Journey is not permitted on the London Underground part.

This is true 7 days a week, even if there is no engineering work.

The barriers at Kings Cross are correctly programmed to give you back London Terminals tickets and they are accepted by the London Underground gateline at Kings Cross and the relevant other stations for this reason.

This information isn't available to the public, but the staff-only Manual states

National Rail tickets which include validity between Finsbury Park and London Terminals are also valid on LU / DLR services between Finsbury Park and Kings Cross / St Pancras Und but intermediately at Highbury & Islington only (Victoria Line / Piccadilly Line), and also between Kings Cross / St Pancras Und and Moorgate (via Circle, Metropolitan, Hammersmith & City and Northern Lines), but intermediately at Old Street only.