If I were you, I would definitely call them tomorrow or Saturday and see what they can do. I find that they are usually very helpful in the guest services centre, and they’ll do whatever they can to help you. If you’re lucky they might arrange you a taxi, or at least ticket acceptance with Scotrail between Edinburgh and Dalmuir (I believe, to make things easier, you can get a direct train from Edinburgh to Dalmuir on the route via Airdrie, but I wouldn’t be sure of how the connections worked). They might exceptionally let you use the Lowlander, but don’t be too disappointed if they don’t (it might already be full, for instance). Hopefully they won’t make you stand waiting at Arrochar and Tarbet for three hours for a bus! (I’d tell them that wasn’t acceptable, hopefully they will have the good sense to see this).