Cabbage root fly only attack members of the Brassica family. Could you manage to get a photo of the attacked roots? We can help better if we know exactly what you are dealing with.
If you have them then your alpines and pot plants will not be affected, unless of course they are Brassicas.

We've only had them for three days and there's no damage to anything that i can see. There must be a good couple of hundred of the little blighters and they seem to be attracted to the Ajuga which is growing rather too well on the rockery.
We visited a garden centre this afternoon to look for some more plants and their Ajuga was covered in them too!!! I pointed it out to one of the staff and asked what i should do about mine....he recommended bug killer.

This doesn't sound like cabbage root fly. Maybe some other small fly like compost flies. If there is no damage after three days then they are probaly harmless and should be left alone. Many flies are beneficial.

They look like a very small bee/wasp type insect,it's Hubby that's been down the garden like David Attenborough identifying the species so he could be wrong......I'll hold off with the weapons of mass destruction as they don't appear to be doing any harm,they don't seem to bite or sting either and fly away from you as you get near them.
Thank you so much for your answers,very much appreciated : )

Sound more like Hover flies than anything else. That is why we like a photograph if possible.

Went to take a photo.....but they've all disappeared as suddenly as they arrived!!!!
I did get the feeling they were almost sunbathing as the rockery is in sun all day but yesterday was a drizzly grey day and they've packed up and left.
Think you may be right about them being hoverflies....didn't know they gathered in "swarms" (for want of a better description) seen the odd one or two at a time but not the amount we had. Oh well...crisis over. Thank you once again for your replies,very helpful : )