I've searched the web and google and it suggests you should be able to
get a charter refund added to the card, alas

https://www.tfl.gov.uk/tube/contacts/refund-form.asp

Only offers the refund as "Credit voucher" - am I being thick or have
they withdrawn the most convenient option - the cynic thinks to deter
claims.

Can the credit voucher be added to the card or am I going to get a
voucher that will allow me to purchase the cash fare only rather then
the oyster fare.

Interesting to note, the only reason I am claiming this refund is the
the tube was screwed I had to get off and get a bus and the station
staff refused to refund my journey - because I had travelled - despite
them not getting me to my destination - what a crock of ****.