It sounds like vine weevil. Have a look at :
http://www.dgsgardening.btinternet.co.uk/vineweevil.htm

It sounds like vine weavil grubs. Take a sample along to the local garden centre for ID ASAP as you should not plant anything in the soil until you either dispose of it away from the site or kill them using a rather expensive preparation. Vine weavil like polyanthus so if that is what was in over the winter that could be the source. They do not seem to bother begonia sempervivums which will stand a bit of neglect. The white stuff (is it powdery) sounds like something left over when the soil dried out.

Thankyou,Wagger and Scotsgran. I have often heard of vineweevil, but never encountered them. Now I am worried that I have brought them home from church and infested my own garden, as I have been planting up a lot of stuff on my potting bench at the same time. Help !

Take heart Rosiebeck. Lots of gardeners have them and live with them as there is little you can do if it very widespread in your garden. I would think it would be far too expensive to treat a whole garden and the neighbours would pop over the fence to visit anyway. They go from being those little white grubs in to flying beetles. They lay their eggs in the soil. The grubs eat the roots of plants they like and they keel over and on it goes. You just need to be vigilant and destroy any you see. If you have them and you see a plant looking poorly have a look at the roots. If possible destroy the whole plant and get rid of the soil around the roots.

You can collect the grubs up and feed them to the birds - they love them and it's a lovely revenge, lol.

I've found some of these 'nasties' for the first time on a Forsythia so they received the sole of my shoe !