Hi Jules,
No, seed potatoes are not just anything lying around. I worked for 25 years for a company that processes around 400,000 tonnes of potatoes per year in Australia... One of our biggest aims was to have the best seed for our contract farmers that we could get... So we had a breeding program just for that..
You see, a potato is a cloning plant... whatever disease the plant picks up in a growing will be carried on the the next generation...(scab for instance lives in the soil) If you breed up the plant over, say, four generations to get a final commercial crop, it may have 4 or more diseases...
So, for best results, go to a nursery and ask for "certified" seed..
Cheers