Hi Archie
I understand your frustration and my apologies for not spotting your thread earlier (Mach 1.5 gave me the heads-up!).
You have discovered the need to set the trigger before installing but, assuming that nothing has been added internally, the only thing left is the arrest of the intermediate sear SR134. This is usually implemented by a collar (Sear Stop Bush W198) around the trig block retaining screw. The numbers refer to the Chambers diagram at
https://www.gunspares.co.uk/products/24531/Longbow/
If all this gets you nowhere, don't panic as they are well worth having when sorted! Give me an e-mail and I will help all I can!
I can send a dimensioned diagram of the internals since I prepare them for all models I strip and note differences, where they exist, in all samples!
Sounds like you have the makings of a fine rifle there! They are certainly my choice for a medium weight barrel cocker!
atvb
David