Cottage pie with sweet potato mash

Made this before going out for a walk and then ate it when we got back and it was really yummy. The mash was a little sweeter than normal though. I bought my sweet potatoes from the 'Continental Shop' which is packed full of delicious and exotic goodies. The potatoes were labelled sweet-potatoes but had a darker skin than normal and when I peeled them they were white inside rather than the usual pale orange colour. I've googled it and I'm not sure whether they were a different kind of sweet potato or whether they were yams?!Whatever they were they made a cracking sweet mash for my cottage pie.