CJB wrote:
ZSL stated that they needed TWO *paper* tickets from 'outside' London
to London - although they couldn't define the term 'outside' nor the
exact destination London station required. Actually ZSL hadn't a clue
what was valid or not, all the women at the ticket desk knew was that
Oyster Cards and Freedom Passes were not valid. So we both had to pay
the full admission. CJB.

Since they do not define "London" the customer may interpret it to
mean "City of London", so any ticket from outside zone 1 to zone 1 ought to
be sufficient (or a station inside the City of London if they really want
to be pedantic).