Although I was starting to wonder whether I was actually going to get a reply to this, it arrived yesterday. The salient points are as follows:
"...I was sorry to hear of the details of your journey and I apologise for the inconvenience this caused. As an apology, I have ordered a cheque for £14.60, to cover the cost of your ticket and the additional bus ticket that you had to purchase, which will be sent in the next 7 - 10 days."
I'm very, very pleasantly surprised by this - overall it was a more than reasonable and thoughtful response, over and above my expectations. Now £14.60 isn't a huge amount to ask for but the fact that they've seen fit to reimburse it without fobbing me off with vouchers or excuses is the main thing as far as I'm concerned.
I'll be writing back to thank them for their response, it only seems right to do so.