Thanks for your response Gary.
My seats are basically black cloth with green piping (photo attached). The piping is coming undone on the driver's door side and there is a pair on eBay which shows exactly the same wear so this may be a weak spot. My pair cost me £40 nine years ago but, as you say, there are not many good ones at that sort of price these days!
Many of those currently on eBay are leather or half leather ("Alcantara" - presumably the design name) but I assume these are from later models. Newton's website confirms that the Mk I model dates between 1995-99 so presumably later models may be a different size or shape. With winter approaching I'm not desperate to get better seats so I'll keep my eyes open for something suitable over coming months.