"BTW is canberra air to air refuelable?"
Well, no, not strictly speaking. But it is my understanding that plans to fit PR9s with a probe for Corporate (to allow them to operate from Ascension and, if memory serves, South Africa) reached a VERY advanced stage, with full engineering drawings etc. and a robust conversion programme planned. In the event an 'alternative' solution was found for that op.
I believe that the drawings were dusted off more recently, not in order to extend PR9 range, but to allow two equivalent sorties (in terms of film footage/tape/targets) to be flown on one pressurisation cycle.
Pressurisation cycles are, I believe, a limiting factor in how long the PR9s can last. What a shame more aircraft were not kept in store when 39 down-sized to Flight strength (as 1 PRU) after Corporate....... Instead three went to Chile (back to Chile?) and several were scrapped.