I was just hoping if anybody knew where I could purchase some late main crop potatoes?
You could try the main seed companies such as Kings Seeds, Dobies, Thompson Morgan, Mr Fothergills. It is quite late now though so I would not hold your breath. You can order potato sets from T&M to plant in containers in August for harvest at Christmas. (New potatoes)
Given the time of year, you'd probably be better off getting an early (they don't have to be lifted early!) as then you'd be sure of a crop.
http://www.thompson-morgan.com/potatoes1/group/potato+%3A+second+cropping.html
Try:
www.suttons.co.uk