It depends on your budget and how much you want to do yourself but Quick Fit seatbelts near High Wycombe seem to offer most options. If I understood the rough telephone quote correctly they will fit inertia reel units from scratch for about £1300 + VAT. There are pictures on their website and it looks a very good option.
If you just need the third mounting point I have found mountings that look ideal for the top of the rear wheel arches. They will need to be welded in.
https://www.ebay.co.uk/itm/175923664831
I can help with measurements for location per factory fitment on a 410.
Once you have mountings Quick Fit should be able to supply static belts for around £100 per side.