Boyley
I agree, according to Dave Scrimshaw's present prediction table, number would fit with a circa 1976 Holdsworthy factory frame.
Probably you have, but if not, you should check out catalogues and model discussion on nKilgariff.com - the definitive site for all things Holdsworth!
Do you know if it is in original paint and decals? Holdsworthy would have likely placed a model name decal obliquely on front of top tube? With Prugnat I lugs it has to be either a Mistral, Super M. or a Cyclone. Does not seem to have a Cinelli style fork crown which makes me inclined to say not a Super Mistral. Can't make out seat cluster detail but if has full wrap over seat stays and Reynolds DB tubing this would point to a Mistral. If neither than would point to a Cyclone model which was basically an "economical" Mistral. Pics of seat cluster, Reynolds decal and fork crown would be very helpful. Do you know size of seat post?
Doug