Jane's World Sailplanes (1978) description of the HP-14C:
"The HP-14C is a Slingsby development of the HP-14 high performance single-seat sailplane designed by the American R. Schreder. Slingsby had decided to change over from wood to metal when the use of glassfibre or metal was necessary to remain competitive. They took the basic design of Schrederís sailplane and revised it as necessary to meet British requirements.
The V-tail was found to be unsatisfactory for several reasons and it was therefore replaced by a conventional fin with all-moving tailplane. The 17-metre wing span was increased to 18 meters and the whole wing moved back 15 cm (6 in) to give a wider centre of gravity range and more roomy cockpit. The flap operating system was completely redesigned: the original rack-and-pinion wind down mechanism was felt to be unsatisfactory since the aerodynamic loads at high speeds made it impossible to apply effectively. A pneumatic system was therefore used, with two jacks to lower the flaps to 90 degree at VNE, otherwise the flaps are operated manually at normal speeds up to 92 km/hr (50 kt) with the aid of a bungee. The compressed air bottle (at 1200 lb/sq in) requires recharging after about three operations of the 90 degree flap position.
The first HP-14C was completed in time for the 1968 World Championships. Three were built before production ceased after the fire at Slingsby in 1968."