Because of this thread I decided to reboot my Shub3, just in case.
Good news!
It seems that 350Mb is going beyond mere trials. I had noted the other day, that for the first time ever, that I was getting TBB tests close to 300Mb. Previously I was getting less than 200Mb, with a big difference between the single and multi-thread result. It seems that the local network has been upgraded too.
The increased rigour on ISPs providing advertised speeds during peak periods may have something to do with this too. It's been in the news only this week.