Great that it has the new cocking lever already, Paul.
I forgot you had the 97 too (I sorta remembered you had another HW, but couldn't quite remember which one

). So, yes, stick that one in and see what happens.
I only had a few shots with the 35 and you'll have a far better idea of how many times the issue has reared its head. On those occasions where it cocked, it appeared to do so to me smoothly and without hindrance or obstruction, so fingers crossed that it's just the trigger issue. And quickly answered if you try it with the 97's trigger. Hope it works as that will be problem nice and easily diagnosed and a cheap fix. As you're happy with the rifle's shooting manners, personally I wouldn't bother stripping it to investigate piston etc if the trigger does the trick.