You're going to get loads of ideas, some simple, some a bit more complicated.
(I'm going to waffle now, bear with me, these are my own experiences...)
In my own quest to sort out interference I nearly went nuts, figured most of it was down to my neighbours and luckily calmed down before I started doing anything stupid
Years ago everyone complained we were interfering with their TV reception etc. due to our CB radio antics.
"Must be you.... You've got that stinking big aerial up!", they'd say...
Nowadays it's us on the brunt end of interference with just about every electronic device kicking out spurious radio signals
You've done the basic tests, (switching off electrics etc.), and still have the problem.
You can go mobile and get away from the interference or even move to an out of the way location, but Diss is hardly a metropolis area is it?
Most of my noise was coming through the mains wiring - even when you switch off the electrics it's still coming through, an RF "sniffer" or simple AM/MW radio proves that. But, if you had a communications receiver you'll see that most, (not all, the "new" and WARC bands etc.), are actually free from interference. That's because the power companies had to start filtering that hash out! Unfortunately CB frequencies aren't catered for.....although you will find mid-band/EU/CEPT is actually less affected.
So, a mains filter might help but I didn't find much of an improvement.
Hmmm, unless you know what you're doing I personally wouldn't start mucking about with them. You could make your antenna a fantastically efficient lightning rod, your mains protection unable to function, (think street grounds and neutral bond looping), and even pick up noise from the ground!
You can buy them, (usually expensive), or make them. I'm lucky I was able to make one. What a PITA that thing was!
Some people claimed to have good luck with them, most example videos show them to be great, (usually heavily doctored to get you to buy their product), but what a pain. You fiddle, tune, change 'noise' antennas and on it goes. Get one frequency clear and the moment you QSY you have to fiddle with the damn thing again! Yep, not the 'magic' solution.
Working with your neighbours.....
Yeah, good luck with that one. They don't care about your radio 'hobby', they won't let you start fiddling with RF chokes on their TVs etc. and it won't take long before they swing it back around to you and blame your CB for their Wi-Fi dropping out 'sometimes'.
Heck, I've even heard people blame CB for making their 12 year old, 300,000 mile washing machine play up!!
I have a neighbour with a spangly solar panel roof...damn thing could transmit across the cosmos to other intelligent life...do you think they care about my radio? Ha, not one flipping bit!
Not sounding good is it?
It's well known that different antennas have different noise responses, by design or their efficiency increasing your signal AND noise floor. You'll get differences of opinion on types of antennas, manufacturers and makes, loops, end feds, deltas, on and on it goes. Most of us get stuck in that quest for the 'holy grail's of antennas...and why not? For most of us it's flipping fun and we grow the proverbial 'antenna farm'. If that's for you, sure, give it a go.
OK, pause, pant, deep breath...we're getting somewhere.
This so far is what I went through. Not much fun is it? So what did I do....
Turned my RF gain down and switched my noise blanker on!!!!!!!
No way?!! Yep, flipping worked and amazing we miss the simple stuff! If your RF gain is cranked all the way up, you're also pulling in all the crap. Got a weaker signal? Seems counter intuitive to turn the RF gain down doesn't it? But you'd be amazed how their signal suddenly jumps out at you. I won't go into technical nonsense but your radio often works better with that gain knocked slightly down.
Noise Blanker, (NB), Automatic Noise Limited, (ANL), or any other fancy thing a manufacturer calls it. These can be surprisingly effective but sometimes with a catch....on my own 40 year old Yaesu it's NB is fantastic, so long as I do the opposite of what I've just said and turn the RF gain all the way up! Again, technical nonsense explains it but if you 'boost' the noise the blanking circuits work better.
Flip...after all that I found a simple solution
I know, can't be true....but give it a go, would've saved me a lot of trouble if I'd had that little tip.
I went from S9 of hash on the triple 5 to S2 with that simple exercise. What a dumb f**** I am!
Anyway, (waffle over), keep at its there's bound to be more suggestions but worth trying simple stuff first.
(Like the 'clarify' control, not all of us are bang on frequency and that can often improve a DX contact).
Oh, before I go. A friend had similar issues....we tried everything....until I noticed the rusty old PL259 hanging out the bottom of his antenna. We changed that and amazingly his noise virtually disappeared! Like I said, the simple stuff
Let us all know how you get on.
All the very best to you,