I do remember a colleague saying that Labour had actually promised the 35% and it just happened to fall on the Conservatives to carry it through.
AFPRB had found we needed 35% to get back in line. Mrs T in opposition promised (and once elected, delivered it) in one tranche. Calaghan had offered to pay it over two years (which they'd done several times before). Not many service people (or their Mums, Dads etc) voted Labour in 79....