Saunier Duval Thelia 623 boiler / shower issue?

Saunier Duval Thelia 623 combi boiler

The weathers nice at the mo so the cental heating has been off the last month or so. Since then I'd noticed that gradually the water in the shower was getting colder and colder and now its just cold. Something ive also noticed is when I would turn the guage to cold, normally the power of the shower would get stronger, but now the pressure stays the same. The boiler pressure guage is on zero. When I turn the shower on the boiler lights up like normal and the bathroom/kitchen hot taps still work fine (athought go hot-cold-hot-cold etc).

Any advice, and if its a pressure issue, do you know how I get the pressure up. I've been told it should be on about 1.5.

Many Thanks