I backed up my phone then did a settings | about | reset
Phone then booted up, but could not access the internet with vodafone sim (due to not knowing the APN), so I had to put in an o2 sim. Then I restored my last backup and all my data came back, along with Internet Sharing so it seems to work.
Once I started it up with my Vodafone sim in it it mentioned different tarrifs - Vodafone Postpay and Vodafone Prepay, so it might know the new Vodafone APNs now - haven't check that bit yet.
The only downside was having to stick ?10 of credit onto a PAYG o2 sim :-(