I think I have found the noise, coming from a house in the next street, I would estimate about 100 metres from my QTH. Certainly the awful PLT noise is receivable outside the house even with the telescopic antenna collapsed.
I should emphasise that (Luckily) this noise does not affect the amateur bands, but in parts of the broadcast HF bands it is quite bad. For example on 11620Mhz this afternoon it is S9.
At least if I decide to report this problem to Offcom I can now give them a general idea where the noise comes from. It is a pity people are using these devices and I wonder why they are in use. Surely a wireless system would be better for distributing internet around the house?
Here is the PLT noise on the Digpan waterfall with my TS450 on 11620Khz.