Dave
The date is quoted in a 1993 Aviation News article on trials Comets, whether it is correct or not I cannot confirm. It does make sense for XV147 to have been at Pershore for the Nimrod avionics trials as that was what it was intended for, its Avons being fitted with additional engine driven generators to supply the large power demand. XV148 on the other hand was the aerodynamic prototype and that had Speys.