Finally ending up purchasing one from that well known internet auction site. Item was advertised as a Gates part (so I had buyer protection to fall back on) and paid £42.50 for it and free postage.
On looking at the tensioner it is marked INA Slovakia. So, a phone call to Gates technical support at their head office in Denver, Colorado. Their very helpful tech support chap told me that they only sell parts made by OE suppliers and INA was one of those OE suppliers to GM.
You don't know who makes what nowadays!
Still happy with our Dualit toaster though!!!
