As a DIYer (still trying to learn) to another DIYer I would offer a little advice.
The heights are probably irrelivant, as was posted earlier, replacement sockets were probably put in the holes in the skirting left by the old ones to save replacing all the skirtings.
If the fittings are all are all from the 70's then you're wiring may date from the same period - but it may not. If you can find some exposed cable (maybe at the fuse box or if you can take the front off a socket or light switch - remember to turn the supply off) then if it is black rubber then it will be very old and a full rewire, if it is grey PVC then it is more modern and I personally would not be overly concerned - some other people here may disagree?
Also checking if the lighting circuit is earthed may give you a clue to the age of the wiring. To do this unscrew one of the ceiling fittings (again remember to turn off supply). If there's the green and yellow earth wires then all is good, if not then your wiring is probably older. Maybe one of the sparks on the board here can tell us when it became the norm to earth lighting circuits?
If all checks out then you're probably in the same boat as anyone else buying a vaguely modern house. Assuming the rewire was done properly then stuff should be okay and I personally wouldn't worry.
I had a similar situation when I bought my place. My place seems to have been rewired in the 70's - cream light switches, grey PVC cable, earthed light circuit so I was happy.
That said, I have since come to beleive that the rewire job was not done too well. as I have redecorated I have found cable protruding through plaster, connections in fittings not screwed tight, one faulty socket in the house and a faulty socket in the shed that I've disconnected from the circuit anyway because it was clearly not done right in the first place.
The moral of the story - a fairly modern installation may seem okay at first glance and not need a full rewire, but it can often be worth checking for faults because you have no way of knowing how good the spark was that did it.
If you do have to rewire then it will be a messy job as all cable runs will have to be chased out then filled and redecorated over.
As far as your loft conversion is concerned the wiring will no doubt be put on a seperate circuit(s) so it will just be a case of getting that wiring to your fuse box. I'm not sure about what issues there may be (if any) regarding your fuse box not being able to cope with the new circuits but I don't think fuse boxes are particularly expensive if it needs an upgrade. If this is the case then no-doubt the spark who does your loft wiring will let you know.
Sorry for the long post, hope it's useful
