I want to replace my 25 year old gravity mixer shower and install a pump to boost water pressure. The bunf that came with the shower talks about needing a surrey or essex flange. I may already have one but I need to know how to identify it. The hot water feed for the shower leaves the cylinder at an angle, almost at the top next to the immersion heater. It has a great big nut on it like pictures I have seen of flanges. So is it a flange? Or just a big nut?