Today I decided that Matty's old seat had the better clips and adjusters, even though the cover was in poor shape, thinning and with small tears!
I took it to pieces, put the straps in water to clean them and washed the seat frame of its grime and sand! Then flatten out the old cover by removing the elastic edge, and use it to make a pattern for a new seat! It looks much to big spread out like this!
and here we have Mary's soft new cover, complete with the 'seatbelt snugglies' she requested!