Descale Unvented Pressurised, Water Cylinder - Poor hot water pressure

I currently have an unvented pressurised system at home for my heating. Unfortunately, living in the Thames Valley area, I've got hard water issues at home, and as a result scale built up in the water cylinder. The hot water pressure at home is nowhere as good as it should be (since it's a pressurised system, I'd expect close to mains pressure). Any idea what to do? I've been advised to descale my water cylinder but spoken to a few local plumbers who've said they won't do it for an unvented system. Really struggling now - apart from replacing the cylinder itself (too expensive!), any other ideas please? Much appreciated. C