I found the PDF no problem - I thank you for the link! Printed it out last week. Lots of useful stuff, most I already knew about but the biggy in it for me was that whilst it discussed the KOPS (knee over pedal spindle) method, it lead me to learn about the COG method (centre of gravity) although I don't think they mention it by name. You know, the balancing comfortable without using your hands to hold yourself up. I think it makes far more sense and COG method positioning is possibly one of the most valuable things I learned... I went on to read a whole bunch of stuff about it from
Steve Hogg which was useful.
By the way, I valued, read and follow all info and links posted throughout thread (well most of 'em). Both sides of the argument were useful in my decision making.
Anyway, the reason I'm here is to post an update - I wasn't even aware this thread was still rumbling onwards!
As I'd previously mentioned, I'd tweaked a bunch of stuff (seat higher, cleats forwards, new saddle, angle etc). Managed about 3.5hrs / 90km on Sunday. Back still hurt towards the end, but didn't become too noticeable until around the 3hr mark. Couldn't have gone much further, but that's a big improvement for me. I'm also wondering if it is fitness/stamina related. AS I get more tired I wonder if I sit on the saddle heavier, putting more load through my back?
Anyhow, that ride made me decide to move saddle a few millimetres higher again, and also forwards a bit (still getting used to new saddle). No issues at all on a fairly zippy 1hr ride this morning... new position felt different but good. Perhaps more powerful/direct. It's hard to describe. Looking forward to trying another long ride soon (maybe my first 100km).
So, to conclude (at least for me - as I'm sure this thread will keep on rollin') I think if I didn't at least appear to be making gains myself I would still be wondering if I should go for a bike fit. However, from my original question, the answer wasn't a definitive 'YES, get a bike fit' from a majority... there was a fairly even split. I appreciate it was worth it for some, but not so much for others. Not conclusive enough for me to go for one right now though. Possibly if I'm still struggling next year if I haven't managed to fix things further.
Thanks!