OK Rubberband2, you want some details, here goes.
No doubt you are well aware that in the early 70s the Mk3 could have either a 301 or a 302 series Avon, the 302s were known locally as the 'GT'. At the same time, there were what were known as 'tuned' jet pipes. A Mk3 with 302 series engines and tuned pipes (the 'GTS') could be very a slippery a/c, lighter and with less dag than the Mk6.
So, my opportunity came by chance, nothing planned. The brief was for a pair to do some PIs against a jammer then individual AAR and back to the jammer.
The No2 failed to get airborne, the never to beforgotten WHEEEEEEE-PHUT.
I did some PIs and then filled to full on the Victor, literally just as I left the tanker the Canberra broke and had to go home. What would you do with a Lightning full of fuel and nothing to do?
My initial plan was just to get to Mach 2 which I did, then what do you do---like anyone else I pointed it up and just held it. Didn't get as high as Dave R or Brian C, even with a full Mk3 I simply didn't have the gas but I wasn't all that far off.
As an aside, I've seen a photo taken in a Mk3 cockpit with 85 grand on the altimeter so on the right day, anything's possible.