In order of likelihood:

1) If these have been outside it is most likely frost damage. If so, all you can do is hope they kick back in.

2) They look like they are still in small pots so it could be lack of nutrition. They want to be in a big pot of ericaceous compost with a good feed of ericaceous fertiliser. Or better still, plant them out in a sunny position with plenty of peat added to the soil and also feed them.

3) Could be just magnesium deficiency in which case a feed of epsom salts wouldn't go amiss.

In the event of 1, frost damage, doing 2 and 3 wouldn't hurt anyway.

I was just about to write my answer but Sarraceniac has said it all. When you repot it or move it just check for any bugs or weevils in the root ball

I got them 2 weeks ago in much smaller pots buy 1 get 1 free so i have since repotted them wth ericaceous compost but wth no improvement,i have kept them in my green house, i also have got some sulphate of iron feed for ericaceous plants wether it has magnesium ingredient i dont know but option 2 & 3 seems good to try....fingers crossed thanks vm sarraceniac ;-)

If they haven't been looked after properly over winter then you will, unfortunately, have been sold them too late to get them under glass. I suspect that that is why it was BOGOF. As you say, fingers crossed. And, no, sulphate of iron has no magnesium. So a weak feed of epsom from the chemists will help.