You saw her again after that? - did she not ask you for the money you owed her from the previous visit?
Maybe it's just me but I would contact the girl as soon as I realised & get the money to her somehow. (Providing the punt was worth it).
Yes i have seen her several times since then but it was a good 12 months between forgetting to pay. Guessing she must have forgotten!
Oh come on! Have you never heard of Never look a gift horse in the mouth!!!!
In that situation you may think different. Morals are one thing but when it comes down to it, you can either keep your £150 for another punt or go back and give it to her....