Checked all valves and they are all fully open. All hot taps in the house are fine its just the shower so Im assuming its because of the routing of the piping going up instead of under the floor then just up to shower. Basically, there is a pipe coming out of the centre of the top of the cylinder. It then tees off, one going down under floor, the other going up to the loft. This is the vent line with the shower teed off it. There is approx 1m, maybe a bit more, from shower head to top level of tank. Even less with the rainfall shower as its nearly touching ceiling.