Well, SMS is definitely working, so thanks again, GuyAdams. Getting 3G internet access may have to wait until I can find a carrier who's prepared to sell it on terms I can agree to...
Does anyone have insight into using USSD codes, e.g. to check on PAYG balance? I've been trying a Sainsburys PAYG SIM today, which is based on the Vodafone network - but so far, I haven't risked any more money on buying credit, after the Three Mobile / Hutchison 3G experience.
According to the Sainsburys mobile FAQ, the only way to check the balance is with *#1345# - but that doesn't work, as far as I can tell, i.e. USSD codes in general don't work on the Nexus 7 3G.
Did they ever work, in a previous version of Android? I'm still not too opposed to the idea of just reverting to an older version, if that works, and turning off the system updates forever. Apparently some data plans can't be used without them. Using PAYG without ever knowing the balance would be a total pain too. Vodafone has a 2345 number for doing it, but they don't allow that to work for Sainsburys customers, and the Sainsburys online account doesn't appear to have a way of checking the balance either.
I have a phone that the Sainsburys SIM worked in, but I had to snap the edges off to get it to fit in the Nexus 7 - i.e. turn it from a normal mini SIM into a micro SIM by popping out the micro SIM part from the outer part. That's not an easy process to reverse... I could maybe do it with glue, but there's no easy way to make it interchangeable between the two devices.