The picture for

GWB23 - British 3" Stokes mortar 

appears to be of

GWB19 - British 18pdr and crew

and

GWB24 - British 6" mortar

appear to be of

GWB20 - British 4.5 Howitzer and crew

also, the 1915-18 Corps HQ pack lists 4 x 6" Howitzers and crew, should this be mortars, or is GWB24 in fact 6" howitzers rather than mortars?

Quite right.  Normal service, as they say, will be resumed shortly as I get the correct images into place.

GWB23 is definitely the 3" and GWB24 the 6".

Move along there, nothing to see here, (cough, cough)...

All is now well in the World of British Mortars.  The right pictures are in place against the right codes...