Q: sim failure following ios 7.1.2 update

Hi.  I'm hoping someone here can help.

I have an Ipad2 which was replaced for a faulty ipad early last year by a UK Apple Store (Lakeside) - it is therefore unlocked.  I attempted to update to 7.1.2 last weekend;  unfortunately the ipad froze on the apple logo, so after some time I did a full restore, updating to 7.1.2 at the same time.

I have been happily using a giffgaff sim for the last year;  following the update to ios 7.1.2, I am now getting a sim failure.  When connected to itunes it shows that the sim is unsupported and I should use a compatible sim from a supported carrier.  I have tried a new o2 sim and get the same result.  As a last result I have tried an old vodaphone sim which, interestingly, works flawlessly.  I have tried changing the APN settings for GiffGaff, but this makes no difference;  it is as if the IOS update has locked the ipad to vodaphone.

I have restored the ipad twice and have reset it after each apn change - unfotunately nothing is working.

Any advice would be gratefully received!

Kevin