What is the best way to test out a niche bike when shops rarely have them in stock? I've emailed a couple of bike shops about Surly bikes - One in Oakham and one down in Richmond. 

The one in Oakham seems to be understocked and the other one is not responding. 

The one in Richmond doesn't have the bikes in the store in my size and I believe it is the biggest distributer in the UK.

How can I know if the bike will fit if I can't test one out when the bike is niche and shops don't often stock my size (XL 58+)
I have two main approaches:


look for someone who owns one who is nearby. With recumbents and tandems this is often easier as there are clubs and online forums for people who own them. For unusual-but-not-too-unusual bikes you may have to ask questions on local cycling forums and hope for the best. With fora like strava encouraging people to list their toys this can be easier than it might sound. Most people are fine being approached with "that's a great bike you've got, I'm interesting in buying one and would like you to rave at me for a while about why you bought it".
When I want to test ride something very specific, I'll ask the importer or manufacturer for help. Often they can say "we shipped exactly what you want to shop X", so at least you have some chance of finding it by ringing the shop. Or they may have one available that they're willing to ship to a local shop for you to try.
if it's just sizing, I find measuring myself, getting the measurements of the bike I'm interested in, then comparing the bike I want to a bike I have works ok most of the time. Even if the geometry is a bit different I can at least assure myself that the bike I'm ordering is close to the right size. If you want to get really keen, find a shop with a bike fit jig and set that up to exactly match the bike you're buying. It'll take time and you'll probably have to pay for it, but if it's worth it to you, do it.