Sounds suspiciously like Vine Weevil larva, Alan. Try googling this for pictures. Are they in pots of plants? If so, I would suggest unpotting the plant, cleaning the roots of compost and dumping the old compost in the rubbish bin. Then repot again with fresh, clean compost. Provado do a vine weevil killer to water on the soil but if the little beggars are already in there they will be chewing on the roots of your plants and need to be got rid of asap!

There is a picture here;

http://www.thegardenersalmanac.co.uk/Data/Vine%20Weevil/Vine%20Weevil.htm

Bulbaholic helped me with this problem last year, and I am still finding them.I have had to replace all the compost in over 40 tubs and baskets so Alan be ruthless to eradicate them. If you enter vine weevil in top right hand corner and click search ,you will find all the remedies.Good luck.

The adult vine weevil cannot fly so as an additional precaution you could try smearing something like vaseline around the pots. The adults eat evergreen leaves and you can sometimes find them in the evening. They go in search of nice soft soil with juicy plants to lay their eggs. I have learned the warning signs now, and as soon as a plant starts to look a bit sick, I upend it to look for vine weevil grubs. Sometimes I manage to catch them before too much damage is done but often the plant cannot be saved. I have just lost a favourite potted clematis . I find the Provado quite good in giving protection and yes, definitely get rid of plant roots and all the compost.

Just to mention, I also had brown headed grubs, but after watching them for a minute, they started moving on front legs. They turned out to be chafer grubs. (lifted as many as possible, put them in a hig sided tub, the birds seem to love them.)

Also look up Armillotox on google. It strongly smells of gas tar. It is said to be effective against vine weevil eggs. The manufacturer is not allowed to advertise it as a pesticide, otherwise it would be banned by EU rules. It is labelled as a soapy cleanser effective on moss and algae. Dilutions for treating weevils are given on their website. Don't use it before you have a BBQ though. I'm going to try it this year to try and keep them off my Heuchera.