I spoke with John from Heron Security and Sport (they are now the agents for Merkel here) and he was very helpful and provided lots of information.

I will examine and see if I can fix my problems myself first. I pulled out the weight from the buttstock last night and sure enough on of the mounting screws had broken and the other was badly bent. The screw broke off below the timber - may see if an easy out is available at this small size. The weight is a tight fit and has an o-ring on the end - it was in fact air tight.

There's another weird plug like thing in a hole above the weight - not sure what it is.

Here's some photos:

As far as checking the lock system (John suggested there may be some debris causing the RH sear to not engage fully) I came to another obstacle - there is a third box plate screw (think its a wood screw) underneath the trigger guard and the slot head was already damaged. I'll have to drill the head off, but haven't gotten that far yet.

It had been a while since I pulled the foreend and barrels off and there was some flattened soot(?) on some of the close mating surfaces - needed steel wool to remove it. My RH barrel was causing some action closing issues and this may contributed - under the extractor was otherwise not too crudy.