Thanks for the recent responses. Family duties complete, I’ve managed to have another look at the battery. As far as I can tell, each of the (4) cells is charged to just over 3v. I’ve reduced the balance charge rate to 1.5A (from 2A) and that seems to be working - 20 mins and counting. Either later tonight or tomorrow I’ll try connecting the 857 again. Rather worryingly Fake Britain is showing an item on the dangers of fake Lithium batteries as I write this 
. There’s a lot to be said for the technology of 2 yoghurt cartons and a length of string!
In the meantime I’ve ordered some 45A Power Poles, 5.5mm bullet connectors (to modify the ends and save altering the battery connectors direct) & a 8.4Ah battery to run the 857. If I can get the 4.2Ah battery to perform, that will go with the 817.
SUCCESS!
Balance charge completed and 857 fired up (on rx only for test). I still don’t know what I’ve done differently (other than reducing the charge rate). Maybe there was a slightly imperfect connection somewhere, that was cured during the fiddling.
Anyway some useful advice for future battery work so thanks to all who’ve contributed.
73s
Just saw this as email notifications have never worked for me here.
I love the 857. Very easy to use. No issues. Programs easy w CHIRP as well. Added a cheap CHI TCXO unit. Would love to eventually get it in the truck and upgrade to something else in the shack. The Icom 7100 however is a better radio IMO. IF stage filters, USB audio, SD Card recording, bigger screen, etc.
Sounds like you got the 857 so hope you are enjoying it!
73
Clifford
The system logs say they emailed you 7mins ago (2121Z) and that 0 email bounces from your registered address have been received. That tends to suggest the system is emailing an address that works.
email is correct, no spam filtering at all, verified grey and black lists clear. No email. Thanks for looking into it.
Clifford