Mr Thant wrote:
Cash in the Z1-2 ticket (~£650), buy an annual Z1-3 (£1208), cash it
in August (~£900), buy a new Z1-2 (£1028), cash it in in January (~
£600). Get the timing right and the net outlay by you is well under
£100 and you don't lose your Gold Card benefits. Obviously it requires
you to have some spare cash available.

Alternatively, the suggestion of a Bowes Park-Finsbury Park rail
season ticket is a good one (£46.90/month or £114.60 pro rata -
remember you can buy arbitrary length season tickets and Travelcards
at the monthly rate).

I'm interested by this suggestion as I'm in a similar situation- annual
Z1-4 on Oyster expiring August, will need zone 5 from June/July. Now I
originally thought I could do what Barry suggests downthread-effectively
adding Z5 for the remaining period. Out of interest I asked at the
Victoria TfL info centre yesterday, and couldn't get a sensible answer
because they were so confused about Oyster season validity on NR
(questions like 'Are you commuting to a Tube or mainline station in Z5?'
Er, it doesn't matter.) But having read this thread, I suppose I could
save quite a bit by going for a Shortlands-Bromley S paper season, and
just moaning every time I have to get it out for the barriers...

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Current nearest station: Clock House