Hey all,
Good news...my back is feeling a whole lot better!
Whilst trawling through YouTube videos for fixing lower back pain, I learned about the Quadratus Lumborum (QL) back muscles and how you can work them to relieve back pain...I looked further and tried several different stretching techniques and hey presto, the pain is almost entirely gone! This Livestrong video contained a couple of effective stretches that worked for me:
View: https://youtu.be/py-qXCuFzaA
I also followed some advice and lowered my saddle by 5mm, and that also felt better during / after riding, although I've only done a 60km ride so far. Fingers crossed I might have fixed the issue....let's see next time I do a century ride, hopefully this weekend.
Finally, I've added some lumbar support to my desk chair and a foot rest, so I'm more comfortable while sitting.
Cheers for all the feedback.