Mounts that fit the No.4 fit the No.5 as well. You know your No.5 has some value to collectors if not irretrievably altered, right? So if some bubba hasn't already drilled and tapped it for a scope mount, don't be the first.
S&K, B-Square, Armalon, ATI and Cad-Technik mounts don't require any drilling or other permanent modifications. Personally, I like the Cad-Technik but they have drawbacks, as do all the others. Most require removing the rear sight. Marstar sells a mount that looks much like the Armalon, at a lower price, and they don't have a brand on it.
As far as I know, they all work for Weaver type rings, so any scope for which you can find rings to fit that go on Weaver type mounts, can be mounted on your No.5, but you'll have to check the length and the the bell diameters for clearance and adjustment for eye-relief.
With my Cad-Technik mounts I have mounted Weaver K2.5s and K4s on a No.5, and an old Micro-Trac variable 1.5-4.5x, and a Bushnell Trophy 1.5-6x44 (30mm tube) on my No.4s so they would probably go on a No.5, too. The Cad-Technik does mount the scope high, (one of the drawbacks many people cite for this mount, but I find it workable with a Beartooth comb-raising kit which doesn't permanently alter the buttstock, from Russells Sports in Calgary or online.)
Other cons with this mount are that as far as I know, you'd have to order it from the U.S. (try Brownells) which might require more tiresome paperwork than when I got mine, and it is more expensive, I believe, than any of the others. Some people have complained that theirs didn't hold zero, but I think you'll find some examples of that complaint with some owners of all the other mounts, and my two Cad-Techniks have held zero on four different rifles. What I like about it is that you can keep the rear sight in place and it is a two piece mount that allows you take the scope off with one piece to use the iron sights or to allow removal of the bolt for cleaning, and it will return to zero when you put it back on.
The other mounts are simpler and cheaper and mount the scope lower than a Cad-Technik, but I think all mounts on an Enfield put the scope higher than is ideal. I think they all attach in place of the rear sight, so you won't have your iron sights while you have a scope, but you can return it to original configuration. I believe Armalon, which is in the U.K., will ship to Canada, but as I said, Marstar's no name mount seems to be the same for less money. I haven't tried either but I have heard people say the Armalon is good. I'm not sure who sells the B-Square, S&K, or ATI mounts in Canada.
