Hi Graham
Something i haven't made for ages. Yummmmmm
My mum makes hers using a can of Salmon but if you can get some fish fillets you can mince your own fish. Whiting works good.
She adds a good cup or so of mashed potato without the milk and butter, an egg, breadcrumbs if the mix is abit soft and doesn't hold together. Salt and pepper and a diced onion. Roll them in more breadcrumbs. Bought are fine but i prefer to make my own-- think the flavour is better. The secret is getting them to hold together and not break apart when cooking.
Hope this helps.
Ronnie