Yes - I'm a bit idiotic all round
Thanks for that link to the thread about making the TP-Link dongle work nomko.
But what I did in the end was just to go and buy an Asus N13 dongle - which claimed on the box to support linux - and having a good amount of confidence in Asus gear in general.
However even though the Mint13 kernel has a driver for the N13 in it, and it recognised it straight off, it kept regularly dropping its connection off - and the only way to reconnect it, so it seemed, was to reboot the whole OS
So after a bit of googling it seems that the Asus driver is a bit on the flaky side and the best way around this problem was to check that it had a Realtek rtl8192cu chip in it - which it did - and to then download and install the driver for that chip from the Realtek website. Which I did. Now no more drop-out problems.
In case anyone else looks at this query the solution is in the post by "ahallubuntu" halfway down this link:
"Ubuntu Forums > The Ubuntu Forum Community > Ubuntu General Support > Networking & Wireless
[SOLVED] ubuntu 12.04 rtl8192cu connects with bad ip"
And, yes, one does need to blacklist the three lines that the poster talks about at the end of his excellent post (= by "ahallubuntu") otherwise the odd dropout still happens.
And now the "Realtek Wireless LAN driver" shows up as being Activated under "Proprietary Drivers".