These 716 patent guns some times were provided with a tool that locked onto the cocking rod and used the knuckle bearing surface as the 'pivot' to pull the rods forward, one at a time. I have seen a couple of these tools at auction over the years, usually bearing an ebony handle and very nicely made, but most seem to be languishing in the back of desk draws!
The fact is that the snapping off of English sidelocks was considered unnecessary by most makers: they often gave instructions for how to do it and supplied snap caps as a matter of course but they didn't actually encourage it.
In fact I have see very few antique springs'settled' by leaving the gun cocked, I have seen dozens with broken springs from being fired!
If you are really worried about it, get a Purdey and store it broken down!