[Nikkithefoot]

I have in the past let some Charlottes grow on and become much larger spuds and used them as maincrop.  Can't say how well they would store longer term as they were all eaten by early October.

[Auntiemogs]

Thanks Nikki, I shall have another look at John's guide. 

[homegrown]

I read about it on here before so I tried it
No idea if it works 

[Mark's Sussex Allotment]

Fair enough