I think there are 3 types of worms, ones that live in the leaf litter and organic matter above the soil, ones that live in the top layer of soil and those which live much lower down which are the only ones that move vertically through the layers. In gardens with poor soil there isn't the habitat for these various worms so it can look very depleted so we need to improve the conditions for them which is why the advice of adding organic matter (food) will help. In the autumn you can add leaves as these are pulled down into the soil by the deeper living worms.
Thanks so much everyone for your help. We have a Hotbin composter and that has, as Dovefromabove says, has composting worms that appear almost like magic. We also have quite a stash of spent mushroom compost so we will dig some of that into the soil and also empty the Hotbin and add the compost to the soil along with the spent mushroom compost and some used compost from potted plants. Hopefully that will all help and I will also try Jenny_Asters' tips on digging holes and adding peelings to those and hopefully some worms will appear and if not we will buy some earthworms. Really grateful for all the comments as I thought it highly unusual not to have any worms and now I realise why!
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In gardens with poor soil there isn't the habitat for these various worms so it can look very depleted so we need to improve the conditions for them which is why the advice of adding organic matter (food) will help. In the autumn you can add leaves as these are pulled down into the soil by the deeper living worms.