There is parking in Marsden near the railway station (follow the cobbles). It is then a pleasant walk along the canal to the centre. The on-site parking is reserved as disabled spaces. The visitor centre is mainly aimed at children but there is enough to interest adults too.
The cafe is very good although it can take a while to receive orders as everything is freshly prepared. A good range of food, including a lot of vegetarian options. I would definitely recommend the boat ride into the tunnel. The guide is very knowledgeable and the experience is quite unique.