As suggested by Tim Horn I tried painting the building with Halfords primer once I had stuck it together. This shows my interlocking brickwork nearly worked but the end of the stretcher finishes about .2mm too shallow behind the adjacent wall. To make it work I'd need thin the wall sections down to something like 2.8mm before gluing up the joint. I'll try this next time.

Also on Tim's advice I then tried painting with an acrylic mortar colour Citadel Karak Stone from Games workshop and then wiping off with a tissue/cotton bud and IPA. I had more success than my previous attempts but I didn't realize I was actually scrubbing too hard and probably could have done with a better coat (or preferably multiple thinner coats) of primer.

Onwards up the learning curve!

David