Yes, the guidance in the IVA manual would be appropriate. That will result in fitting the proper mountings. Anything less is dangerous, and a MOT fail if the tester can see that it has been done improperly. But as for legality, only a court could decide. If you don't need rear belts, because of the age of the car, it is hard to say that improperly fitted belts would actually be illegal.
However the insurance companies don't need a court ruling to give them the absolute right to refuse to pay any claim if there was an undeclared "material fact", as it is well founded in insurance law, and if they found improperly fitted belts they would be fully within their legal rights to refuse any claim. So you would have to tell your insurer, who would either refuse cover altogether or charge a greatly increased premium if they heard of belts that were fitted improperly.
Frankly, for £80, compared to the consequences of not having them, and the risk of having an accident, I would not hesitate to fit them properly. NO OTHER INVESTMENT OF £80 WILL INCREASE SAFETY BY SO MUCH. Consider that, in a frontal accident with the rear passengers not belted, they are likely to be thrown forward and kill themselves and the front seat occupants too. The annual fatality rate plunged when wearing rear belts, when fitted, was made compulsory.
