I have a pre-pay oyster-card, purchased and registered on-line.

I do not have a convenient Tube station near me, I don't use the Tube or
trains - the only use is for occasional bus journeys.

It seems there is no mechanism to top-up my card, without considerable
inconvenience.

You should be able to get your Pre-Pay topped up at a "Ticket Stop" (i.e.
your local newsagent that sells bus and tube tickets) if you come across one
of the 2,300 or so equipped for Oystercard. Would that help?

G.