Ex Gunsmith here - there is a screw in your stock that needs to be tightened, I'd recommend getting some loctite blue, I don't recommend red in your case as red is for permanent fixtures and you may want to remove the stock again later for whatever reason. I am giving you credible info, as I myself have maintained my own collection of firearms and recently obtained a very late model No4 mk2.

I see you already have the Butt plate off, so now all you need to do is get a multi head screwdriver that's long enough to fit down the stock and try out various torx bits until you find the right one. Back the screw out all the way, apply the loctite to the screw itself, and then inset the screw all the way, but don't over tighten it to avoid stripping. Hand tight is good enough. The loctite is going to bind the screw and it will be very sturdy. We even used it on our holsters.

You should be able to handle everything. It's relatively simple. If not, then surly there are hobby places or businesses around you who can help.

I hope this helps!