Just to point out that if the PIR recommends a rewire, then you probably didn't need a PIR to tell you that, the guy probably had a pretty damn good idea after 15 minutes before even getting the test kit out, and the rest of the time gathering evidence of this to put in a nicely bound folder to tell you that... (of course it does become usful if you decide to ignore his recommendations and instead ask for remedial works on the defects highlighted)
Of course, it could be one of the few that looks alright until you start testing, but they are not in the majority... you usually can tell
Classic example, place wired in the late 1960s, wooden framed wylex board, undersized bonding not very much in the way of provision of points, insulation resisance was down, lighting earthing seemed hit and miss with some points wired in T+E and properly earthed, others wired in DI singles with no earth in, but an earth borrowed from the ring, some spurs off spurs, etc, The guy also wanted a lot of points changing round in the kitchen that he was totally gutting, and also the bathroom lighting redoing as he was gutting that as well, in my mind after you have put in a new fuseboard, bonding, found the IR fault, fixed the other issues, done the alterations, you may as well have rewired it and have a brand new installation...