It just doesn't work!
PLC systems usually are of limited range. Here in the USA we have these X10 devices that allow houses to flip lights and outlets on & off. These work quite well, as the bit rate is probably a bit les than 100 bps (yes, one hundred bits per second). With these rates, they can time the transmissions at the zero crossing of the power line, and they work quite well with limited range (inside the house).
The PLC people (it is called BPL, broadband over power line) here think that they can use power cable (14 gauge wire 1/4 inch separation) like we all use Cat-5 wire that is tightly twisted for multi megabit transmissions. It just doesn't work. What happens is that the power line acts like a nice antenna (the adapters would probably work better if they just had an antenna connected instead of trying the power line as an antenna).
The other problem is that they think that they can get broadband signals to work. Guess again! the aggregate data rate of a bunch of users is WAY beyond the published capacity of the system. It just dies on the vine.
So, it is neither broadband, or over power line. The interference happens everywhere, and the more users that are there, the worser it gets. The limited tests here in the USA have all gone bust, and our FCC (you call it Ofcom) has all but pulled the plug (pun intended!)!
The sooner this dies, the better!