Signal Noise & Buzz

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Hi all, I need some advice on some bad buzzing issues I have with my setup at home.

I have a Vox AC10 that I bought back in February and it has been fine until a couple of months ago when it began creating a lot of noise. Kind of a buzzing, hissing sound - a bit higher in pitch than the hum that a single coil would usually create. If I crank the gain on the amp it gets very loud.
I thought there was something wrong with the amp but I tested a Roland MicroCube in the same sockets and that produces the same noise. In fact I would say that the Cube is more susceptible to it.

To complicate matters worse I recently acquired a Les Paul 60s tribute with P90s in it. At quiet volumes it produces a fair amount of hum. At high gain the hum is very loud. I've always had single coil guitars so I can handle some hum - but this is way too much.

Observations:

Amp @ Low Gain. Guitar plugged in: hum that reduces significantly when I touch the bridge or strings. Turning down the volume of the selected pickup almost eliminates the noise.

Amp @ High Gain. Guitar plugged in: very loud hum & hiss. Touching the guitar does little. Rolling back volume reduces some of the buzz but it is still loud.

Amp @ High Gain. Nothing plugged in: loud buzz/hiss.

I've checked the earth with a tester plug - appears fine. I tested almost every socket in my house and they were all fine too. 
I tried turning off every electronic device that I could (pretty much everything but the fridge) and it made no dent in the noise whatsoever.

My knowledge of electrical devices and networks is pretty limited, but my guess is that the main source of the noise is the ground connection. I live on the top floor of a terrace conversion so i'm not sure where the earth goes. The strange thing is that it was fine several months ago. Construction works on new flats began at the end of my road about 50 metres away shortly before I noticed the noise. I would've thought that the site would be using its own generator as a power supply though and not the grid.
I suspect this noise is only being increased by the P90s. 

Any advice on how to better diagnose this issue is very much appreciated. I would also like to understand how effective the following is likely to be:

1. Shielding the control cavities of the guitar
2. Using a Hum Debugger
3. Using an isolator like this (http://www.orchid-electronics.co.uk/trans.htm) or the Ebtech Hum Eliminator (although as I understand it these are mostly used to combat Earth loops when running two amps together)
4. Selling my soul and finally getting a guitar with Humbuckers in


Also as an aside, I noticed in a practice room at the weekend playing through a very large Marshall that rolling back the volume from 10 towards zero created buzz. No buzz on 0 or 10 but buzz when either volume knob was dialled back. I understand this can be because Les Pauls use quite high impedance pots but it was fairly noticeable. Is this noise normal or is it a symptom of the issues causing the above?

Anyway, sorry about the long post. Any advice is much appreciated.
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