Most probable to be 12 each x 0.6" dia 7-wire strand, in a multi-strand duct, grouted. MUTS of between 1725 MPa and 1860 MPa. Stress-relieved, and possibly NOT low-relaxation.
Based upon your 'handle' name I assume you are based in the UK, and also assume the bridge is too. From the early 1950's until late 70's/early 80's the Freyssinet system was licensed to PSC Equipment Limited in the UK, and I do not have any PT system data for the UK, however, I do have the Freyssinet PT system data for the US and in 1972 this is what they detail for their strand systems:
It is most likely that the US and UK systems are identical (especially the anchors and other hardware) but the grade of strand will be your biggest unknown.
As hokie66 suggests, give Freyssinet UK a call, or get a copy of Prestressed Concrete Designers Handbook by Abeles et al, and I think that text details the various UK PT system date from a historical perspective.
I hope the tendons were correctly grouted! Several UK bridges around the same period of time had some systemic problems with inadequate grouting (grout bleeding and inappropriate techniques) that were across several system suppliers/installers.
