Derek was wondering what had caused the problem. It is of course, disappointing when you eagerly await the harvesting of a fresh crop.
You insert your fork well back from the root of your spuds on the end of the row, so as not to damage any with your fork, eagerly anticipating the taste of new potatoes boiled with a sprig of fresh mint.

The crop appears out of the soil and suddenly the sense of achievement turns to one of disappointment when the crop is not healthy.
Holes are apparent and when you cut into your precious tubers you find tunnels. What could be the culprit?
Slugs do make holes such as this, but once inside they tend to eat out the potato and so if cut open you find caverns have been created. More likely in this case would be wireworm, which only leaves tunnels.
In Derek's case I tried to reply fairly swiftly, but since then I have been inspired to find out more.

Wireworm is the larvae stage of the click beetle. On investigating the whole subject I found some interesting facts to share with you.
Chemicals to fight wireworm were taken off garden centre shelves a couple of years ago. My day job at Aylett Nurseries in St Albans keeps me in contact with the retail market.
Such products as Bromophos that were soil insecticides have been taken off the market because the chemicals were too persistent. They were not being broken down by soil bacteria before reaching waterways.
There is now no retail product that will protect against wireworm. I have heard that the only control for wireworm in seed potatoes was to plant some early varieties that would attract the pest and then on digging up said tubers you could destroy the pest attached to the potato.

Hardly inspiring – while you wait for all this to happen your gardening friends are up and running with their crops.
I note that in the trade there is a method of attracting the pest in order to monitor it, so on a small scale it may offer some control.
Research has shown that the wireworm is attracted to carbon dioxide that would be given off by potatoes as they respire underground.
Growers have special traps, but I think you could make your own.
Cut down a lemonade bottle, retaining the bottom. Drill holes some 3-4mm diameter, the same size hole as a wireworm would make.
The experts use pure ryegrass seed to put into the container. The wireworm is prevalent on grass and likes to live on the roots.
Note this, because if you leave grass growing in your veggie patch this can attract the pest.

For your own use, buy some grass seed with ryegrass in it and you will easily find it on the shelves for creating hardwearing new lawns.
Add a little water and bury just below the ground. Within a few days the seed will start to germinate, giving off presumably a special flavour of carbon dioxide. It should attract any wireworms in the area and you will be able to lift out your trap from the soil and destroy any you find.

Most infestations are found on sunny south facing slopes. Presumably the soil is warmer earlier here and so life is easier for wireworms. I hope all this has helped – and good hunting.