OK, so yesterday it all went pants - I took a picture and my phone wouldn't open the gallery at all. I tried a few things but ended up rebooting the phone. It booted fine (suggesting the ext partition was fine), but the SD card would not mount, and the phone said the card 'may be damaged'.
I tried plugging the phone into my PC in the hope of being able to 'fix' the card - nothing. I took the SD card out and put it directly into my laptop, and it appeared, but nothing again.
Finally I decided to format the card - that should have left the ext partition in place and only format the 6.4Gb of other space. First time all looked good, but the format failed - second time it was going to format to 7.4Gb, so all was not well - the ext partition was gone.
Eventually I partitioned another card, factory reset the phone and started again. I simple brought back the Titanium backup I had, and I was back up and running after an hour or so. However, I learnt my lesson, and I now have a new Nandroid backup and a new Titanium backup.
Below is a screenshot of the RAM I now have to play with...
I have now ordered another SD card in case. I believe if I partition the new card, do a Nandroid, put the new card in and then restore, all will be fine. I will report back.