ST(A)420 was kicked off in 1988, thus IMHO that is when the Nimrod MR2 Replacement saga started.
I'm not sure when the official staff requirement kicked off but I do remember glossy brochures of the P7 (LRAACA) being passed around 201 Sqn in 1987 with the line that we (the UK), were buying into the programme and was intended to be a lead customer alongside the USN.