Without going into detail, we can confirm that arranging a reservation for a descent in the mine is a nice example of Belgian surrealism (cf. Solivagant's review). But after a few atempts we made it to Spiennes and we can also confirm that the local staff and guides are very helpful and knowledgeable.

It seems that the parking close to the site is meant for the elderly and/or disabled, but you can park at the Rue du Petit Spiennes - Rue de Nouvelles intersection which is only a 5 minute walk to the site.

The tour starts with an explanation above ground and ends with a descent in the mine. This last part takes some time since everyone has to get in/out of safety gear. In the mine, three layers of flint/silex are clearly visible.

In principle, taking pictures is not allowed but - as we have also experienced in a number of other Walloon WHS - the guides mentions that as far as they are concerned taking pictures is no problem (and that they only have to say it pro forma).

Ps: the first Sunday of the month, the entrance to the site is free of charge.