How do you know you haven't got any? Have you had a good deep dig?

The 'overgrown with conifers' suggestion could be a significant factor, Phil. Conifers suck all the nutriment out of the soil and put nothing back; your soil is probably very dusty. Is this a new garden to you? The number of conifers should probably be seriously reduced and some humus put back into the soil.

Manure, compost and LIME! Earthworms and other soil invertebrates need calcium for their metabolism. Acid soils like those in conifer forests are almost devoid of soil invertebrates.

We used to garden on an extremely acidic sand dune (dead easy to dig and when we built the extension to the house I got the sand for the concrete from digging it out of the garden) we had no worms and despite adding 20 tons of animal manure every year for 10 years when we left there were still very few worms to be seen. However the soil was very fertile, but it lost its fertility very quickly and the worms departed.
Buddleja is absolutely right about the way to help the worm population.

So, are most conifers very hungry plants? I should need to fertilise my thuja as often as possible?