Having gone through the T.F.L. telephone rigmarore to speak to a
customer assistant, only to discover that I was No. 26 in the queue, I
thought I'd get a reply much quicker here!

I have a weekly Zone 1-2 Travelcard on Oyster (not by choice, but my
local newsagent gave an Oyster card as the only available option - I'd
have preferred a paper card).

For 2 days, Wednesday and Thursday, I need to travel 1 stop out of zone
2, to Balham (by rail from Clapham Junction). I don't have or want "pay
as you go", and I don't think this is valid on national rail over that
route anyway.

How do I go about getting a day return 1-station extension on my Oyster
card? Will they be able to deal with this at Clapham Junction? I don't
want to spend 15 or 20 minutes queuing tomorrow morning only to find
that they can't sell me what I want anyway.

And, if the worst comes to the worst, and I end up having to start my
journey at Clapham Junction without buying the relevant extension, will
I be able to buy it at Balham or will there be a penalty fare to pay?

Many thanks for any answers!

Marc.