Thanks - I mean the water when you open the hot tap is only lukewarm and certainly not red hot - I normally mix hot and cold water to get a good temp whereas now I only use the hot water tap.Do you mean the cylinder primary return is at 50°C? That's what I would have assumed, but then you refer to the shower, which is obviously on the secondary. You wouldn't want the 2ndary to be anywhere near 80°C, if that's what you mean.
Does it behave different from the old cylinder?
Have you tried throttling the rad pipe, at any valve as it's a 1-pipe system? To force the water through the coil. As somebody said, an air vent (manual) in the horizontal pipe would have been a good idea, but pushing all the water through the coil should push the air round the system and out via the vent (can't remember where that is).
There is no way to throttle the radiators as there is one big flow pipe which then tees off for the hot water cylinder and then continues for the rads. Previously all the boiler flow water would go to the cylinder - Now it seems that the radiators are taking it as if the flow pipe is not getting round the cylinder
Is it an idea to adjust the pump speed? Here is the pump at the boiler - Is this set to max or min