If it's a gravity system without a pump, you're going to struggle to get this to work. From the Esteem website, it looks like your shower needs a minimum pressure of 0.2 or 0.4 bar, depending on the model. This equates to a minimum head of water of either 2m or 4m respectively, and that's without taking pipe losses into account.
In a two storey property, with the tank in the loft and the shower in a ground floor room, you might just about get it to work OK if it's the 0.2 bar version, but any other combination is going to be less than satisfactory.