For a change I didn't go sim-free as it would have cost me more. Even cost me less to buy out the rest of my contract and 'refresh' on O2.
E.g.
My contract is £27/month (note only 1Gb data)
Sim free Z3C £344
Over 24 months = £992
Upgrade is £28/month (2Gb data)
Early termination/'Refresh' £81
Over 24 months = £753
I couldn't find any equivalent sim-only plans which come to less than I'm currently paying, or certainly not enough to make it cheaper overall.
Could have got it a few quid cheaper elsewhere via cash back redemption etc, but would have had to faff around a bit more.