Thanks folks for all the replies.
The bike was built up for me by Thorn's and I had to wait several weeks for it, so it wouldn't have been sitting around on flat tyres for ages. Also I don't think they have much of a shop-front so it "shouldn't" have been in the sun.
It arrived middle of last November and only had time for a couple of short runs to confirm that I liked it before the weather turned really atrocious. The tyres were fine then - I checked them.
I got it as a
60th birthday present to myself

for fun use and keeping it for best, so for weather/work/family/injury reasons didn't ride it again until April, then not at all in May, hence the low mileage. It lives in our bike shed and not exposed to sun, and I checked the tyre pressures regularly.
I thought that cracks around the bead were normal - it is the gash that bothers me. I have never had that in new or even middle-aged tyres in a lifetime of cycling. Also that front tyre has a lot of fine radial cracks all round on both sides that I haven't photographed. The rear tyre only has the usual cracks around the bead.
I could send it back to Thorn's but I can't prove that I haven't mistreated it. Maybe I should contact them and see what they say. Otherwise I think I will glue the strand and put a rubber patch on the inside just in case, and swap it to the rear for the time being.
I haven't knowingly maltreated it but our road surfaces are so bad in some places that maybe a loose chunk kicked up? It has been on a Thule roof-rack a couple of times but I have never known that to damage a tyre. All our bikes are treated with loving care in the shed and there is not a scratch on this one.
In any case this has put me off these Tourguards so I need to think what to use in future. I think there have been several threads on this - time to start searching.