I remember that, it was reported on BFBS Radio and was probably 1976/77 and they were the last to be fired in Germany. The countdown sequence was 6,4,3,2,1 they didn't use 5 in case someone misheard it as the word "Fire"
I saw some live firings at Hohne in either 1971 or 1972, we were taken from Soltau school to watch. They were frighteningly loud when they took off.
Honest John is in the Field of Fire at Firepower, the Lance in in the Cold War Gallery that isn't currently open to the public, but you may be able to arrange a special visit if you contact the museum in advance.
I was there and saw it too- and have some photos we took of it in the air somewhere. Cost Â£5,000 each missile I seem to remember they announced over the tannoy- a lot of money back then- am I remembering that figure right?