I looked at the photo of the sumpter pot when it was first posted and had my 
doubts.

All of the drawings of sumpter pots which I have seen in the literature eg 
Percy etc have thick circular rims which sit on brickwork.  They are 
hemispherical in cross-section or less than a hemisphere (not sure of the 
term but plano-convex lens-like).  They are similar but much smaller than 
the melting and refining pots used in the modern lead and tin industry and 
about the same size as those used for making special solders.  The 
cross-section gives a large area for heating, if a fire is used.  There is a 
larger cast-iron pot on the site at Nenthead which looks to be around 5 tons 
lead capacity and which has, no doubt,  come from the Pattinson refinery.

The article on e-bay is quite different in that it appears to be thin-walled 
and would hold too much lead than is needed - it looks to be about 1 ton 
capacity which is nearly a shift's output.  Normal practice was to dip out 
the lead ingots as they were being produced - see photo of Rookhope smelters 
in Raistrick and Roberts 'Life and Work of the Northern Lead Miner'.  The 
shape would give more heating on the bottom and this would be more 
appropriate for a hot-water boiler.   The lugs on the side are clearly 
intended to rest on the brickwork surround and act as a support and while 
they would be OK for water, they look a little flimsy for lead.

However, Raistrick (Lead Industry of Swaledale and Wensleydale Vol 2 
Smelting Mills) shows a sumpter pot which he says is from Marrick Mill.  It 
is somewhat like the one on e-bay and has lugs on the side.  It is quite 
different from the traditional pots used in the lead industry, which I have 
described above. I have heard doubts expressed about Raistrick's pot and 
whether it was what he thought it was.

Finally, there are a few copper water boilers around doing service as flower 
planters which are very similar in shape and size  to the e-bay and 
Raistrick's pots but have a wall thickness of only about 2mm.  Thicker, 
cast-iron counterparts were cheaper and were common in houses in the 
industrial towns of the north, where they were referred to as 'coppers' or 
'set-pots'.

I suppose the best way of deciding would be to conduct a metallurgical 
examination of the inside surface and see if there is any lead 
contamination.

Regards

Richard Smith