Rich.
I was on the Killhope underground tour a months ago - yep it does have a bit of fibre glass about, but well worth having a trip underground.There is a nice bit of original stone arching with a bit of original lettering, nice working water wheel with a bob attached, mine truck, mock up of miners working in a stoop etc. The place is very wet so wellies are a must have item, preferably your own as those supplied by the museum are usually a bit damp.
Ouside there is a large working wheel, with jiggers etc, along with a working dressing floor, mine shopt / stables etc. The museum houses a very nice collection of minerals and spar boxes, well worth seeing.
Nenthead underground is also well worth doing - was their last year - but you need to check with museum for opening times. This underground has no fiber glass on display - well that I have spotted!
John L.