Question Author
How can I thank you all enough!
I checked the manhole; clinically clean almost!
I found an unopened 450g bottle of "Drain Magic" by chance, and applied it as instructed (it is actually 92%+ Sulphuric acid, H2SO4 solution and very powerful); after a suitable interval, and with little obvious sign of free flow, got to work with the shower hose, then left everything to settle.
Have just now flushed the loo.........hey presto! And what a relief! I'll save these posts for reference......you never know!
Once again, thanks for so much good advice.Cheers, Mike