Your saddle is (or was) what's called a "mattress saddle." They were made by Brooks, Wrights, Middlemores, and others. Could be Raleigh made them in house and branded them with one of those names. What's missing is a layer of padding (sometimes made of horse hair) with a fabric (often fake leather) on top.

Replacement saddle covers were commonly sold in bike shops. Here in the US the common brand was Mesinger, and Mesinger saddle covers still turn up on eBay occasionally.

Looking on eBay now, I don't see any. There was a blue one on a while back; here are its photos so you can see what I'm talking about:





Mesinger was an American company, and their saddle covers look great on a Schwinn. I am sure some UK manufacturer made something similar, and I'm sure you can still find them in the right old bike shop. That's your best bet. Good luck!

It is also possible to recover them yourself. You'll need to do a little bit of sewing. I haven't tried putting real leather on one, but I've thought about it. It's a crazy idea (I like it!). In fact I think I'll try it, just to see what becomes of it. If I do, I'll start a thread about it.

Here's a link to the closed listing on eBay|
http://www.ebay.com/itm/NOS-Vintage-Mesinger-Bicycle-Blue-Seat-Cover-Schwinn-Middleweight-Monark-Higgins-/311929692444?hash=item48a075611c%3Ag%3AVy8AAOxyhotSNhxt&nma=true&si=%252FcQ1GYjF5q10hsFbKQBll7AWbE8% 253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557"]