Difficult to diagnose without being there in person, but as others here have said it’s more than likely an air lock. If you’ve bled the rads try bleeding the pump
You need more 22mm throughout, maybe even some 28 near the boiler depending on how big your house is, pipe as near as you can to each rad in 22 then branch off in 15 to increase the flow, I’d also look at zoning the 3 floors, better overall control.
Electricity kills, you’re clearly out of your depth here pal so it’s time to get someone in.
Taking wiring advice from an open forum is asking for trouble and easily confused.
Does the existing radiator warm the room comfortably? I’d Check if it’s correctly sized in the first place before you start thinking about bigger/smaller.
Lol, same here, I have a 2010 VW Caddy which be had for 6 years but it’s getting a little tired now and my wife has a nearly new VW golf. I kinda like VW’s, they’re a tad more expensive but very reliable in my experience.
Will easily convert to unvented cylinder and conventional boiler, or you could pressurise the system if you want, both a straightforward job as almost you need is in the Boilermate cupboard, all you need is to get to a drain for the unvented cylinder.
There’s a load on the estate where I live...