Andrea; wrote:

There is always a guard present on the train but he/she never
materialises to check tickets.

What times do you travel? When I've travelled on it there's almost always
been ticket inspections (although as I tend to travel almost the entire
route that increases the likeliehood).

I have heard dozens of people say that
they never buy tickets as there is no deterrent factor (many stations
are open and unstaffed, e.g. Leytonstone High Road).

Erm yes - but if it's a case of "buy the ticket on the train" and no seller
comes along what can you do? There isn't a machine at Wanstead Park (my
local station) and I understand this is the case at most stations. (And for
that matter Forest Gate-Wanstead Park isn't a valid out of station
interchange so I doubt you could buy a ticket there.) So is someone fare
evading if they have no opportunity to buy a fare?