Didn't see another thread on this so here we go.
Excellent new resource released by the BBC:
http://www.bbc.co.uk/proms/archive/
This would have saved me a trip to the BBC Written Archives in Caversham when I was researching the Proms 1955-1970!
I can now find out what day it was I heard Paul Hart's Concerto for Guitar and Jazz Orchestra in 1988, or the Bliss Piano Concerto in 1991. Super stuff.