Sussed it!

For anyone else who experiences a similar situation, check the horizontal screw in front of the trigger blade, which is locked into place with a nut and pushes the trigger blade far enough back for the sear to engage with the piston.

The screw looks like the trigger adjuster on a Mk 1 Airsporter/Club but appears to have a different function on the Haenel. When I initially reassembled the 49a, I did not thread the screw in far enough before locking it up with the nut. Ideally it should sit with the nut almost flush with the front of its mounting block. I have even seen the nut placed on the other side of the horizontal screw, as here: http://www.co2air.de/wbb2/attachment...chmentid=42786

My thanks to Josie who questioned the screw's position and who was initially told that can't be the problem by me assuming it served the same function as the adjuster on the BSA!

Thanks also to those who took the trouble to PM/Email me with suggestions and offers to take their models apart.

John