Thanks all
I've been reading a bit about them, and apparantly they lay their eggs in September, in the same place that they hatched from. The grubs don't burrow down very far in the soil. So I could remove my plants, dig up as many of the grubs that I can find before the flies hatch. . Once they do hatch, cover the bed in plastic so that they can't lay eggs there again ,and treat the whole garden with the mocro worms to reduce numbers as far as possible. HOPEFULLY this will be enough to prevent a reacuurance next spring. As long as I don't find any evidence of them in the bed then I could try putting my semps back in.
The alternative would be to find nice pot/troughs to display my collection in and plant the bed up with something else. But I'd have to do a bit of googling to find out if there's anything that they don't like.
I know which one would be easier, but I don't know if I'm ready to give up the idea of my semp bed just yet
