well i took some really good advice and drilled the trigger block from below the stock line (thanks DAVEP!!)
i then removed the drill from the chuck turned it and inserted the shank into the hole- PRESTO!!! the trigger block complete was free
the low power problem was mostly (imo) due to the bucketful of grease that some pil**ck had daubed in, it was everywhere-worst of all it in front of the piston head
the hollow bolt (part 43) was (and still is) very tight so possibly, the aluminium trigger block has worn away, or some muppet has had it apart and forgotten to reinstate the gripping washer(more likely!!), the steel threaded portion (part 42) has much play in it, about 2mm-which is about the thickness of the gripping washer
i don't have a socket/box spanner that will fit into the recess to remove the hollow bolt to explore further so i'll take it to work and see what the fitters have got.
ps, thanks to all that contributed- the gun may be back to full health yet