wait until you have healthy leaves before giving them any more feed. To be honest I don't feed mind other than a little bit of slow release in the compost when I plant them up. I have some in the greenhouse and some outside. The ones in the greenhouse fruit about 3 weeks earlier.
If you think they have been grown in a 'hothouse/polytunnel' then pop them in the coldframe and then plant them where you want them to grow.

I have the slow release pellets which you push into the soil next to the plant should I use them?

There is normally several weeksworth of fertilizer in commercial potting compost - most have slow release granules in already that will last quite while.. Until your plant are growing strongly they don't need any at all. More is not always better and you can overdo it. I would not feed them at all until they are big healthy plants, and not then unless they begin to look a bit starved. You wouldn't give a small child endless food when he was too young to cope with it... you can kill with kindness.

Re the question of how often, that fertilizer will last at least six weeks before applying again but for strawberries I would use a high potash fertilizer if I was using anything

But please don't worry about it. Gardening is supposed to help with stress not cause it,and if you do nothing at all but make sure they don't get bone dry they should be OK.
Best of luck with them and let us know how you get on.

Thank you they have had a good soak in the rain( hasn't stopped since yesterday lunch time) I am going to stick them in my cold frame for a while I will post pics of the big juicy berries soon fingers crossed😊thanks again