Further update - my squeaking / chirping under acceleration was starting to get more frequent, so had a look at this useful thread:
Auxillary Belt and Tensioners
From the links in the above thread I concluded it was the aux belt slipping. So bought a new belt from local motor factors (can highly recommend Central Auto Supplies in Foleshill, Coventry), refitted & the problem / noise is cured!
The old belt was a bit cracked but generally looked okay, but once removed I noticed there was a tiny stone that had lodged deep in one of the 'v' channels - I wonder if this was the main source of the noise (sorry forgot to photograph). I guess over time this would wear down the mating rib on the various pulleys & tensioners, so glad I went ahead & replaced!
The job itself isn't too difficult - hardest bit I found was holding back the auto-tensioner (15mm spanner / socket), but with a bit of perseverance managed to get the locking holes aligned & insert a 5mm drill bit. I found it was slightly easier to work through the wheel arch (obviously FR wheel off, but only needed to remove a plastic access cover, not the whole arch liner). Also need to be careful not to get any grease/muck on the new belt while trying to manouver it around the pulleys & tensioners. Recommend take a photo of the old belt routing first so u don't forget! All the pulleys / tensioners seemed okay/not loose, so I didn't bother replacing. Car has done just over 60,000 miles.
Feel free to reply with any other questions. Once again thanks to this forum for steering me in the right direction!