Kat wrote:
It's not an FAQ, and I've asked on the website, but do some of the
learned people here know the answer - I've a number of passenger
charter vouchers: how do I get these added to my pre-pay Oyster card?

Sorry for the delay in replying but I wanted to get the facts right.
AFAICS, you can use anything to top-up your Pre Pay at a ticket office
except Visitor Vouchers, Warrants (other than PERTIS) or Postal Orders.

....or, according to my local station, Customer Charter vouchers. "You
can get your money back from yer Pre Pay vouchers, yer see, and you're
not allowed to do that..."