Thanks for all the kind comments. There are plenty of areas where it's not perfect, but after doing a lot of work to the interior and a wee bit of tidying of paintwork, it's a nice usable family classic.
Yes, all mountings were there from standard. Upper rear mounts were too far forward for inertia belts so I'm stuck with statics. It should have the grey Irvine statics in the front but a previous owner has ditched them for aftermarket black inertia reel belts.
I should have checked before posting, I've looked in my books and see that rear belts were indeed an option from day one. Happy to be corrected, as always
I recall many early P6s, even as a youngster, as parents and family friends etc. had them when they were current and I don't recall ever seeing a series 1 with rear belts fitted from new. Although the Irvin front belts always looked a bit 'untidy' when not in use, many, if not most, of the cars on the road back then didn't have any belts at all.