Don't lose any sleep over it Stuart, Ive been collecting Scottish Militaria, Uniforms, Helmets etc for a very long time and i have only ever come across this pattern of helmet once in the past and that was to the Lanarkshire Yeomanry.
The Airshire on the helmet plate I would have questioned as well, I have lived 38 miles from Ayrshire my whole life and have never once read, seen or heard of it with the Airshire spelling. I can understand your concerns regarding this helmet, the condition looks too good to be true and anything that's been re-gilded immediately puts me on my guard anyway, I would have put this down as a fake also if i hadn't seen the Lanarkshire Yeomanry helmet in the past. This trend for re-gilding thats sprung up recently with several helmets appearing on eBay from the same couple of sellers makes me very suspicious. Why give yourself the added expense of re-gilding in the first place? an original helmet without the re-gilding usually fetches just as much on eBay anyway, i just don't get it.... as a collector i would rather have an original helmet minus the gilding, than a sparkling bright new helmet that's been re-gilded, but that's a personal choice.
Edit:
I did a little snooping around and discovered this helmet plate for sale in a Bosley's sale on the 5th March 2007 with the Airshire spelling....a lesson to us all methinks.
92. 1st Airshire Yeomanry OR's helmet plate circa 1817-27.
A fine and scarce die-stamped brass example. Star of the Order of the Thistle bearing oval inscribed "1st Airshire Yeomanry". To the centre, St. Andrew and the Cross. Two original lead soldered iron loops to reverse. VGC. (£300 - £400)
See "Scottish Regimental Badges 1793-1971" by W & K Bloomer No..257 for voided example.
Bosleys Sale March 2007