I ordered a stock slimline seat from Walridge 3 or 4 years ago. At that time his main supplier was RK Leighton. As possm mentioned, RK Leighton did not have seat bases at that time.
Mike Partridge from Walridge called about a month ago to say that he could now supply a new UK made seat ( perhaps not from RK Leighton). Cost was quite high at about $800 CDN with shipping.
I declined as the seat on the bike is reasonably OK now, although the foam in the rider area is a bit compressed.
A few years ago I bought a new RK Leighton seat for the BSA Super Rocket. It was about $400 at the time and I thought that was a bit expensive, however the quality and comfort made it well worth the price.
I called RK Leighton recently and the situation there is much the same as 4 years ago. They can provide the seat foam and cover, but do not have a complete seat for the slimline.
Glen