i would grow something else there next year, anything but potatoes.
blight seems to be endemic on outdoor toms, the spores must travel for miles on the wind. ive tried outdoor toms for the last three years (without spraying to prevent blight)on a patch of ground that hasnt had toms or spuds on it before so the blight must have been airbourne. the same plants in the tunnel have survived so its not in the compost, pots etc. my experience is that unsprayed outdoor toms are a waste of space and effort, now have to decide whether to give them up or spray

or maybe cram some bush ones in the tunnel along with the tall ones.