So how else can I get an unvented cylinder installed?I wouldn't necessarily get one with G3 cert, they are some of the worst
You will need a G3 certified installer to fit, commission and notify an unvented.
There are some guys on here round your way that may be able to help.
Is the independent that you first mentioned not G3 certified?
I'm actually quite surprised with your BG quote as its cheaper than I expected it to be. I was guessing in the region of 7.5 to 8 from them.
Jon