[veggieman]

I grew two rows of Sarpo Mira spuds (aka tatties where I live) in a new patch of the garden and got a decent-sized crop from this blight-resistant variety. However, when peeling and cutting the tubers, I find that virtually all of them have dark brownish markings within them. It isn't blight as the potatoes are decent sizes but I don't know what the marks are and whether they are Ok to eat.
The maincrop Harmony potatoes, grown in the next two rows were fine.

Any suggestions?

I can put up a picture or two tomorrow if that helps.

[arugula]

I'd be curious to know the answer to this too. I get this in bought potatoes sometimes, well more often than I'd like actually, and I cut those bits out whilst using the rest of any potato affected. I'm still here... 

[realfood]

It could be Spraing. Spraing, a brown, corky discolouration is caused by a virus, and is again controlled by strict rotation.
Visit this page for photos of potato diseases :-

http://www.potato.org.uk/department/knowledge_transfer/pests_and_diseases/index.html?menu_pos=resources

[veggieman]

I don't think it is spraing, which I hadn't heard of previously, as the leaves looked perfectly Ok.

[Rampant_Weasel]

it may be they have got a touch of frost with the recent cold weather and its just the startings of rot from frost damage.

[Iain@JBA]

I wonder if it is internal bruising?

[azubah]

I had this with Sarpo Myra too. I got a huge crop and we still have a couple of sacks left.
I just cut out most of the brown marks and eat the rest.
It may be a bacterial infection.
When I was digging them up I came across a few tubers which had rotted inside badly leaving them hollow and when I picked them up stinking watery stuff poured out. I think this was due to bacterial infection.
The ones with brown marks have not rotted in storage however.
Could it be due to high nitrogen levels in the soil?? We had lots of manure last year.
I have not had either of these problems before, or in any other variety.