The striped seats were fitted to Mini 1000, Clubman and 1275GT in the late 1970s and early 1980s
http://mk1-performan...k/pdf/79-80.pdf
The GT you have got looks like Denim Blue to me, and for £400 that's a bargain. I guess it is a '79 or '80 as it has a square type chassis plate on the left hand inner wing.
Those other seats you posted a picture of look like Clubman estate seats.