No. PAYG credit does not expire.

I would have said the same about Bus Savers until recently.

Except that the credit on the old bus savers isn't expiring as such, you
just need to have the old tickets swapped for new ones. A pain in the bum
perhaps but no financial loss except perhaps the cost of a stamp or a
telephone call.

(This who were not aware they could be exchanged should see page 14 of
http://www.london.gov.uk/londoner/07...er-june07.pdf).

G.