Thanks for replying. I went on to research this a bit more and came across this on the Cambridge Uni site. (I'm awaiting a reply from Oxford.)
"After this period requests for a transfer of College will be considered only on the grounds of disability a substantial offer of funding from another College or other compelling circumstances."
So, it seems there may be a few circumstances in which a second bite at the College Pool may be possible.....maybe even at Oxford too.