Thanks!!!
Just when you think you've covered everything over the years, something like this comes up.
Just checked my T-Mobile 3G connection to find it set at a speed of 230400; now reset to 921600.
Conversely, my Vodafone 3G dongle appears already defaulted at 921600.
Obviously worth checking for all ISP's.
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PS:
Further to previous, I should add my T-Mobile connection is via an internal laptop 3G modem; standard T-Mobile dongles might be defaulted to 921600. My above mentioned Vodafone account is operated via separate dongle; not laptop modem.
This might suggest a case for checking one's internal modem (if used) and would probably account for the fact that improvements noted came after a reboot, rather than close/open of the connection.
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PPS:
Beginning to wish I'd never started this (!); just tried someone else's Vodafone dongle and it appears too be set at a lower speed.
I guess the bottom line is check all internal modems and dongles.