The Swales SD3-15 was developed from the Birmingham Guild BG 135 Gipsy, the 13.5m span version of the BG 100/12 designed by J. C.Gibson, K. Emslie and L. P. Moore of Sailplane Design Ltd. Manufacturing rights of the BG 135 were acquired by Yorkshire Sailplanes Ltd, who built a batch of seven as the YS 55 Consort. The BG 135 was itself developed from the earlier and very similar Birmingham Guild Gipsy 12/15 project which, like the BG 100/12, was intended to be a low cost lightweight Standard/Sports Class sailplane in which either a medium-performance 12m wing or a high performance 15m one could be fitted to a common fuselage and tail unit. This was to be achieved by special attention to structural efficiency resulting from efficient wing skin stabilisation, with rigid foam cores, and low cost was achieved by eliminating taper as well as twin-skin sandwich or ribbed forms of construction. The prototype BG 100/12, with a 12m (39ft 4in) span wing, first flew on 7 April 1970 and was a cantilever shoulder-wing monoplane of all-metal construction with a V-tail and a large hinged moulded cockpit canopy; a 13.5m span wing was later fitted.

Structurally, the SD3-15 is very similar to the BG 135 and BG 100/12.

Slingsby sold jigs and production rights of their T.53 upon Slingsby’s 1969 receivership to Yorkshire Sailplane which produced an improved version with both nose and centre of gravity tow hooks known as the YS 53 Sovereign.

Yorkshire Sailplanes only turned out three of these all-metal two-seaters, conceived as a T31 replacement for the Air Training Corps only to be rejected due to unpleasant spinning characteristics.