By hand today it felt like with the extra GT85 lubrication in there I could move it ever so slightly more (may have all been in my head), but still couldn't get it. I added even more GT85 and kept going a little more in my head thinking it would be working through. Then....
@gazza82 - wow, so as I remembered I'd put my allen keys somewhere extra 'safe'/sensible (as was using more on my bike than the car with them!) so I decided to try the same approach you recommended with a strong thin drill bit. Nervously I gently turned it, no particular pressure, first time it easily just popped open, no drama (aside from me being in shock and don't think I've ever been quite so happy to smell the waft of air freshener from the car). Thank you so much for suggesting this. Right tool for the job thing I guess and why "normal" physical keys have a greater surface, all about the turning moment.
I then popped the bonnet and 'climbed' back into the boot to get to my jump starter pack. It had enough to set the alarm off a few times before dying, so giving that a charge now and will try again later. At the very worst now I have access to replace the battery, but hoping it shouldn't come to that, although knowing they don't like to go completely flat. The car has already have the larger capacity battery fitted by the dealer.
Thanks all for the input and thoughts here and hope this gives further help/encouragement to anyone else who ends up visiting this thread like I did due to have this issue.