For some reason I cannot find any worms in my gardens soil. I recently obtained a compost bin so the soil should be improving. I have tried adding compost to the soil and adding coffee grains to attract worms but none have come. Would it be worth buying some?
A few years ago I tried vermicomposting. It was a very interesting learning experience. I found that the worms can move about in the medium with great rapidity in response to changing conditions.
I think a couple of rules of their behaviour are: to flee extremes of heat, cold, wet and dry, and they are very sensitive to the location of highly desirable treats and will flock to that location. One day I threw in a couple of half sphere grapefruit rinds. Within a few hours most of the worms had decided to go live in a writhing mass inside those half shells until they were almost gone. Inert stuff like paper, even when kept evenly moist, does not interest them very much. When I dig potatoes in late summer I find very few worms in my garden since they have migrated to lower regions where the conditions are better. But when digging over my garden in late fall conditions are cool and evenly moist so I find lots of them at the surface since they migrated upwards.
So it all depends on the time of year and how much they like the organic matter in your soil. Give them a nice rich house to live in and they won't want to move out.
No, it's not worth buying worms in. If there's a problem with the soil that means they are not there, any you buy in will just disappear anyway. There probably are worms in your garden, but you just haven't seen them; you're in the UK, as am I, and here (in the south east) its been very dry for quite a long while until the last couple of days, on top of a relatively dry winter. When it's very dry or cold, worms go down deep looking for moisture and cooler conditions (or warmth in winter), so if you dig around then, you're unlikely to see any.
You don't say what (if anything) is growing in your garden; assuming something's growing and is healthy, even if that something is just lots of weeds, then the likelihood is worms are present and there's not much wrong with the soil. However, it's still worth bumping up soil fertility and increasing humus levels by the addition of organic materials such as composted animal manures, leaf mould, spent mushroom compost, as well as your own garden compost, so I'd suggest you add as much of that as you can possibly buy in. You can either apply it as a mulch across the top (though the soil should be damp if you want to do that) or actually dig it in; if the soil contains plenty of humus from organic materials, the worms will come too. The likeliest time to see them is in autumn, before it gets cold, but the weather is cooler and damp.
In regard to coffee grounds, very small amounts might be considered useful in a wormery if you're vermicomposting separately, but generally, it's not a great idea to add them to soil - the occasional small amount on the compost heap will do no harm, but not more often than, say, fortnightly, assuming you are not adding tons of vegetative material on a weekly basis.