I'm not aware of any proper (that is, enforceable) regulations in that regard. There may well be some "guidance" published by one of our Government Agencies, or by one of the many "quangos" ... all of which are next to useless (or at least, "toothless") in my opinion.
As I say, the best way of ensuring that you receive proper documentation is to include the request clearly in the initial Purchase Order.
"Shall include one copy of all available technical documentation (service manual, circuit diagrams, spare parts lists etc.) plus X copies of user instructions" ... something like that.
And then refuse to accept the new equipment onto the inventory if the required documentation is not forthcoming.
Don't forget that some equipment needs special servicing software, service passwords and what-have-you. Again, if you plan to service to that level, then that stuff needs to be included up front.
