[Headgardener22]

Check them again HG.  It could be early blight (target spot) rather than late blight.

Aunt Sally, I will admit I'm totally confused by Target Spot and Blight. The RHS August Newsletter said that "Early Blight was not a problem in the UK" (I paraphrase). When I queried them their response was
"Many thanks for your enquiry.  You are perhaps right for the commercial potato crops where many key cultivars are rather susceptible, and they are grown in broad acres under very high fertility, overhead irrigation and there has been a switch from broad spectrum fungicides to materials with a more selective late blight-specific mode of action. The commercial media has reported an increased incidence of early blight in Europe and perhaps the UK, but it is not yet approaching the significance of late blight.
 
However we have no evidence that this is the case for home garden crops where fertility is lower, irrigation less intense, a wider range of potato cultivars are grown usually in a mixture in a small space and copper fungicides (which have a broad mode of action) are applied against late blight.  Rotations have to be shorter in gardens of course which would tend to increase incidence of early blight. There is unfortunately no data on early blight resistance in the wide range of cultivars grown by amateurs, no doubt related to the relative historical unimportance of this disease in Britain, but it is likely that they are less susceptible than the processing cultivars, used in the potato product industry, that are known to be very susceptible.
"
However, looking at this website http://www.potatodiseases.org/earlyblight.html, what they show is almost identical to what I've got on my Sarpo Potatoes, but if its uncommon, where would it have come from and why only the Sarpo's?