eradicate woodlice in enclosed space ie in void in foundations beneath conservatory?

Answer 1

Woodlice thrive in damp, dark places like under stones. They are harmless and do no damage to plants. By improving the ventilation (you do have air bricks in the underfloor area?) the decrease in dampness should cause them to move out. If they're getting into the house you need to seal any gaps around the conservatory floor. That's the best bet - keep them from entering in the first place.

Answer 2

pour some creosote down the gaps- this will get rid of em!