Well 40l is a rough estimate, I ended up taking out some rocks to reduce clutter and increase shoaling space. I presume it's ok for me to get 7 harlequin rasboras as advised earlier in this thread? Peter wanted to know what other fish we could get, he didn't really appreciate hearing that there's not enough room for very many more!
Have double checking the stocking threads, harlies seem to be on the big size for the tank I have. I've made a note of spotted/glowlight/mosquito and bridget rasboras as possible alternatives instead.