I've not used them but their shop is about ten minutes from my house and if I was looking for a second hand saddle I'd definitely give them a go. I've certainly not heard anything bad about them locally and Arkenfield EC where the shop is based has a good reputation to uphold so I doubt they'd lease premises unless they were fairly sure it would work out.
The woman who runs it is really helpful and its a great service. I went to the shop to try saddles but I believe she will happily send them by post. Just ring her and speak to her to see if she's got some that might suit you.