As far as I am aware no law but the IET/BSI recommend that a PIR is carried out every 10 years or change of occupier which ever is shorter. It does not say who should commission the PIR however it says results should be given to the person who commissioned the work.
No PIR has a clean bill of health there is always something wrong. And the results have four ratings and any 1's need correcting but the rest is really so you are aware what will need correcting if any work is done.
For example a code 4 may then say "No RCD on sockets" and this tells you if you want to add more sockets likely you will need to have a consumer unit changed.
However although we should all follow the same rules some electricians will code missing earths on lights as code 1 (Immediate danger) others will code it as code 2 (needing improvement) and so although they will both say the same the codes make one think one is worse than the other.
Now if an electrician is doing a PIR for you to sell the house he does not want the buyer to claim from him so is likely to do a very good PIR missing nothing. The same after the sale in one way but likely will be 10 years before his work is re-checked so may be a little more lax.
However unless something major like no earths on lights then anything found is not really going to change the buyers mind.
The people to ask are the Estate agents and the solicitor. With the sort of jobs a DIY'er may do in a house I would want to see a PIR at some point close to buying a new house. However only person I would trust is myself or my son.
Thank you ericmark , my thoughts were if it holds up the sale or puts people off the purchase, it could be money worth spending, like most things i guess its a bit of a gamble , you pays your monney and takes your chance.