How important is dead wood to the ecology of a forest and how much dead wood should remain?
Dead wood is indeed very important to the food chain, and to the ecology and biodiversity of a forest. Lots of biota depend on the presence of dead wood; many species of insects and fungi only live on dead wood. See for example this study of fungi in Danish Beech forests. 
The amount of dead wood present depends, apart from management, on the type of forest, the climate (and hence altitude!) and the age of the forest. This study provides a preliminary estimate of the amount of dead wood in British forests and might be very interesting to you. This study gives a review of dead wood in European (incl UK) Beech forest reserves.

I hope this will help you in the right direction. If you want any more information, please let us know. And preferably include some more information on the specific forest (type of trees, location) that you are interested in.

EDIT:
I recently stumbled across this project,
 that resulted in a systematic review of interventions that affected the occurrence of dead wood and saproxylic species in boreal and temperate forests.