Thanks for the comprehensive analysis @Jeff-UK - I could almost believe you were familiar with the topography of my local area: aren’t maps wonderful?! 
The sweet spot for the node is actually next to The Link Centre (c1980) - an architecture described as: “The space-age, multi-axial design” on wikipedia Link Centre - Wikipedia
Well I think that particular Meccano-style building might also be a rather good reflector for LoRa, but I’m not discounting the adjacent ASDA superstore also.
I believe the Swindon area is a rather mystical place (have you read any of the novels of Jasper Fforde?) and I suspect the art work installed at Shaw Ridge (“How the Mighty Fall”) actually has hitherto unknown special purposes at 868 Mhz, now I realise that the line of sight to the “sweet spot” EXACTLY crosses this art work.
*Surely that cannot be a coincindence!
And what do we find at the sweet spot itself? A service road called “Link Avenue”.
The plot just thickens 
P.S. I’m keen to try some wardriving around the Magic Roundabout