Hi Phil

Whichever way you choose to fix it, I can't see drain flies having much
interest in this - it's a surface water drain? - that is to say, it's not
soil (****) and this is what drain flies need to survive and breed

I think they are interested in the damp corner in the kitchen which
this crack is causing ... We've had the occasional rat in the past & I
wonder if they are feeding on the now-damp remaining pellets. There's
definitely a stagnant smell associated with this area.

J