Had this about 3 months ago, overhead lines coming into property - 1 cable, just like pme, looking down the street every 3rd or so pole had earth cable coming down it, looking like pme, there was a 16mm earth at the cut out that went inside to the consumer unit, but not connected at either end, there was and old earth stake which was connected, so i'm thinking here we have an old TT installation, that the overhead wiring has been converted to pme but the house has not been, phoned e-on and got directed and re-directed, listened to lots of pretty music whilst "on hold" and the answer??? "we dont actually know what type of wiring we have in that area" !! jesus christ!!! suffice to say even though there could have been a lovely low pme Ze to be had, i left the installation as a TT because at least that way i could confirm the earth arrangement!!
So can sympathise with OP, cause sometimes things aint what they may seem, could be TT could be overhead pme, but obvious lack of any earth would dictate to rod it and TT it, oh and the DNO??? i dont know why people say to call them, next to useless advice in my experience