Paul Scott wrote:
"Keith Raeburn" wrote in message
ups.com...
Earlier this week I travelled from Acton Central (in zone 2) to Watford
North (outside the zones). I held a Travelcard season for zones 2-4. I
therefore purchased a single ticket from "Boundary Zone 4" to Watford
North, and was able to purchase this my origin station. This works out
cheaper than a single for Acton Central-Watford North, and does not
"duplicate ticket validity".

Yes - but you are describing an outward journey. As has been pointed out
already, you cannot buy an 'inbound' ticket from a NR station outside the
zones. Don't know why.

Apologies if I have misunderstood, but if you mean I cannot buy from
(say) Hertford East station "a single to Boundary Zone 3 as I have a
Z123 Oyster", then yes you can, as I do it about twice a week.