Roland Perry wrote:
I've not got one but presumably you got a receipt for your £3 which
you'd have to hand back with the Oyster card to get the £3.

I've had the Oyster since they first came out, so no chance I've still
got a receipt.

I really don't think the lack of receipt would be a problem. You're not
told at the time that you must keep your receipt. If anyone really does
want to surrender their Oyster card and claim their £3 back, call
Oyster customer services on 0845 330 9876 between 8am and 8pm.

Would the "free" 7DTC Oyster also be a pre-pay, for ad-hoc use later?

As Scotboy and Helen have said elsewhere in their replies, *any* Oyster
card is good to go for Pre Pay. (Any *unregistered* Oyster card is also
good to go for any 7DTC or 7 Day Buss Pass).

However I'm going to attempt to read between the lines, and maybe read
your mind! If you need a 7DTC, and you also might find it handy having
another Oyster card around (perhaps to give to a friend or visitor),
then buy a 7DTC on an unregistered Oyster card. After the 7DTC has
expired, the Oyster card can be used by, and permanently given to,
whoever you want for use with Pre Pay, or could even be loaded with
another 7DTC or 7 Day Buss Pass.