What pressure should my water tank be?

It seems unlikely you would have a combi boiler and a tank , it's more likely to be a system boiler

Yes it would be a good idea to get a plumber out as it sounds like the tank is below the height of the shower you are talking about and if so you need to fit a pump to give you the pressure you desire but without seeing the lay out its not possible to be 100%

I trust you mean Tribune UNVENTED CYLINDER, it doesn't make sense to have a water tank connected to combi boiler. I strongly suspect it's a system boiler, not a combi. It is possible that the unvented cylinder on the 2nd floor is causing the issue with your shower. If the cylinder is not supplying enough hot water at a high enough pressure, it could be the reason for the weak shower. The size of the cylinder might also be a factor. A 150L cylinder might not be sufficient to supply enough hot water to the entire house, especially if you have multiple bathrooms or if there is high demand for hot water. You may want to have a plumber or heating engineer check the pressure and flow rate from the cylinder and make sure it is set up correctly. It may also be worth considering upgrading to a larger cylinder or a system that provides hot water directly from the combi boiler.

Unsure what tank you mean but I'm assuming it's an unvented cylinder. If so it possibly needs checked over by someone qualified. This would need to be a plumber or gas engineers who holds an unvented certificate. Anyone without one shouldn't be going near it.