The dial on the hygrometer in my case has been steadily increasing for about 2 weeks now, it started off at the centre of the 'normal' section, and today has gone slightly over into the 'humid' part. Should I be doing anything about this? Thanks!

It's very rare to encounter problems due to humidity fluctuations in the UK, and the hygrometer in your case is likely to be pretty inaccurate anyway (unless you have a rather expensive case).

Yeah, don't worry about it, not in this weather - it's not surprising that it's reading 'humid' in April!