It was pointed out by Gregory.Rotter on cameralabs.com that this camera/lens combination was front focusing. Now because I’m a newbie with an untrained eye I hadn’t noticed this. After a bit of google action I came across a test sheet, downloaded it and ran the test

Here’s the result

Model: Canon EOS 550D
ISO: 100
Exposure: 1/250 sec
Aperture: 1.4
Focal Length: 30mm

Clearly front focusing

And I’m loving this lens

So after another bit of google action I found there was a Sigma Service Centre in Dubai HERE. I gave them a bell and made the journey there to drop the lens off. It will be with them for 14 days

I have taken another test with my Canon 50mm f1.8 to check its not the camera. I’ll upload it and post it shortly