Wondering if it had passed, I put the flag back on the garden this afternoon, intending to move it to another part of the garden should the interference still be there, but this time the band was completely saturated with strong noise. I presume it's PLT but I don't know for sure. Maybe somebody will recognise the patterns in this video I just made.
Even BBC Radio 4 long wave and my local BBC Radio Lincolnshire have lots of noise mixing with them.
Edited January 23rd 2016:
About a week ago, my noise problem disappeared. Since then I have had virtually no noise at all across the bands, apart from occasional daytime bits and pieces, which are usually very weak.
I can only guess that whatever it was may have been a device or, more likely, a power supply which had become faulty, though it did seem like PLT a lot of the time. I say this because the noise became progressively worse over the weeks, peaking at S9+40dB.
I was going to contact OFCOM again, but the bands have enjoyed a whole week of relative silence and, throughout the night time, medium wave has been as quiet as when out and about in the wilds using a Beverage.