I hold a Standard Open Return for Cambridge-Gatwick Airport but my
return flight will be into Heathrow instead this Sunday. Is there an
inexpensive way to get from Heathrow to Cambridge utilizing the
cross-London journey I have already paid for?

The best thing I can think of is to Tube to a non-Zone 1 station where
cross-London transfers are allowed on NR, touch out on Oyster pre-pay
and then use my NR ticket to get back in and head to King's X. I
haven't looked up how much money this will save me, if any, or if it is
possible or even worth the difficulty of having to come to the surface.

Thanks for your help.
--
Michael Hoffman