Thanatar Cavas Siege Automata plasma mortar fitting

[Wibbling]

Hello all, apologies if i've put this in the wrong place. 

 

I've loved the Thanatar model and bought one of the resin jobbies, then the plastics came out and I bought and built the Thanatar Cavas (the mortar one) in plastic. The lascannon version I have fits without issue. 

 

However, when I got to fitting the mortar to the shoulder support, the mortar seemed 2mm wider than the end piece (B25) would contain. It is 'level' with the support. I've two of the kits and have the same problems with both.

 

Has anyone else had this problem? Is it just me? Am I going wrong somewhere in the instructions, missing a step that adds a tiny shim to bulk out the mortar support bar? 

 

Many thanks for any advice.