I wonder why half of them died, as Penstemons are pretty easy really (sorry!!) Bringing them into relative warmth will cause them to produce growth, but it shouldn't look weak as such. If they are too dry or are pot-bound or not getting enough light, then that might result in weak growth. If they are very small and are in small pots, I would think they are better off in the leanto. If they are in large pots (2ltr plus) then they should be ok outdoors. Although having said that, some varieties are hardier than others.

Is there any chance of a picture of these plants please?

if you can be bothered with the faff put them outside in the morning and then back into the leanto at night unless very frosty or snowy. The lean to may be a little too warm at the moment.
As sid says which ones were they as i have several that so far have been fully hardy. [not that that helps you:o)]

I think they are supposed to be hardy. I don't know what variety they are although I do have the names of some of them. One is called Raspberry Ripple. I will try to put up a picture but am not too clever at loading them.
I don't know why they died either. I suspected vine weevil but there was no sign. They had arrived in tight little sleeves which are supposed to rot away but it appears that some of them have not been able to poke their roots through and grow into the surrounding compost.
I'll try to collect the names and get a photo at the weekend . Thanks for your help.
Penny

Did you go back to the company that sold them to you to complain? They may send you some replacements with stronger root growth.

Thanks Andrea, I was thinking maybe I ought to do that.