Is the pressure on Halstead Ace High/leaking shower connected?

Yesterday for the first time my shower started dripping after I had turned it off, and there's now a constant trickle from the hose when I take the shower head off. Coincidentally I've noticed that the pressure on my Halstead Ace High boiler is about half a bar and I'm sure it's normally higher. If it should be higher how do I adjust this? Are the two issues connected or just a coincidence? If not can anyone please tell me how to fix the shower? Many thanks