[heygrow]

I grew some International Kidney potatoes (Jerseys) for the first time this year, but have had disappointing results. They foliage grew extremely large (about 3 to 4 ft high/long) but only had about 2.5 lbs total of quite small potatoes from 12 plants  . On the plus side the do taste great 

This is in contrast to the Lady Christl that on average yielded around 2.5 lbs per plant. The Int Kidney were in a different bed than the LC and it was a newly dug bed that previously had some raspberry canes, but I fed it with Growmore and FBB before planting.

Does anyone have any thoughts why I got such different results from the two types?

[Ivor Backache]

The British Potato variety database lists IK as second early with medium size foliage  and LC as first early.
If you have planted them all at the same time, you should have given the IK another two weeks to bulk up. I don't think there is anything wrong with your preparation. A lot of foliage suggests too much nitrogen.

[mumofstig]

The bed would, probably, have benefitted from some manure or compost; after the raspberries it would have been quite depleted.

[heygrow]

Yes Ivor the IK had a least 3 weeks longer before I dug them and they were just flowering. I could have left them a bit longer I suppose, but as the foliage was so big and getting in the way I decided to pull them up.

Lack of nitrogen seems about right as the other bed that had the LC in had previously had peas and been well composted over the winter and I never had such a good crop as this year from these.