Yep, it's under round about where the third tile is in the external picture.
CT-1 was my FIL's suggestion. How thin does that come out? There's quite a thin gap for it to have to penetrate, and I'm not a tested pro at that, but reckon I can give it a go.
Do you have a multi-tool? You could widen the gap with a mortar busting bit. Might also be possible to slice the nozzle thin enough to get it in the gap, then fill the void behind it.
Some flash primer and flash tape would potentially be easier. It will look like shit but who cares.
Yep, it's under round about where the third tile is in the external picture.
CT-1 was my FIL's suggestion. How thin does that come out? There's quite a thin gap for it to have to penetrate, and I'm not a tested pro at that, but reckon I can give it a go.