Off to the SFR shop in Castelnaudary to buy a prepaid 3G data card (as I had been intending to do all along). To make life easier I had printed off a copy of the relevant page from the SFR website and had only to show it to the helpful lady behind the counter (thank you, Christelle).  

Success, they had them in stock.  Euros 14.99 for an ipad-compatible micro-SIM card and the first 3 days of data.  Plugged in the SIM, gave it a moment to find the network and, hey presto, 3G data! 

Performance? 1574kbps download (pretty much identical to what I get from my 2Mbps landline ADSL at home) and 400kbps upload (better than I get at home!).

Top-ups are available from SFR shops and paper shops and over the Net [Update: no online updates unless you have a French credit/debit card] either on a per-day basis or by volume of data.

We will be staying just (only just) over the border in Spain for the next few nights and I am hoping that I may still get an SFR signal. If not, no problems, the hotel has free wifi.  Allegedly!