tucumseh says that source code access was catered for explicitly in MoD regulations in the early-mid 90s. I'm a bit surprised to hear that. It was a bugger's job getting hold of the source code for C-130J in the mid-90s and it would have been much easier if it had been contracted in the way tucumseh says regulation demanded. Perhaps the regulation was there but routinely ignored. There's no such regulation these days.
Policy promulgated by Deputy Under Secretary of State for Defence Procurement, December 1987. His successor, J M Stewart, reiterated this in Issue 2 of the policy, 14th December 1989.
If validation procedures are to be carried out by an independent agency (in this case, Boscombe)
MoD should secure adequate rights of access and use to design information and code for itself and its agents, while protecting the rights of the originators.
Thus, the policy was extant throughout the FADEC development period and, to my knowledge, has never been rescinded. (Which doesn't mean it is implemented!).