Sounds like Iris Borer (Macronoctua onusta).

Iris borers are the most damaging pest of irises. They will attack all varieties of irises. The adult moth has dusky brown front wings and lighter yellow-brown rear wings with a wingspan of up to 2 inches. The moths emerge August and September. The females lay eggs in the fall in dead, brown leaves. The larvae appear in April or May. They are pinkish in color and about 2 inches long.

The tiny larvae make pinprick sized holes into new leaves, tunneling into the leaf to feed. As the larvae grow, they feed behind leaf sheaths and in stems and flower buds, gradually moving downward into the rhizomes. They typically reach the rhizomes by midsummer. The feeding causes streaks on leaves that appear tan or water-soaked. The larvae devour the rhizomes; just one iris borer can eat several rhizomes in a summer.

http://www.arbico-organics.com/product/iris-borer?gclid=COiy6O-xtccCFdUXHwod4vIOmg

thank you for your help bathgate.

Not sure this moth is to be found in Britain though.

The Iris borer is widely found all throughout Europe including Britain. It also attacks other bulbous roots - onions, tulips, etc.

Strange how it is not mentioned in any of my books and on the Interweb it is shown as a pest of North America.
One lives and learns.
Added.
Just looked this up and it is confined to parts of the USA and not found in Britain or Europe......................yet.

Yes, Macronoctua onusta is not a pest in the UK. Many iris rhizomes have small holes, they are often the remains of old roots and are nothing to worry about.

"Nothing to worry about?" They are dead and full of holes. This sounds like the work of the Iris Borer.

Except that the questioner lives in the UK where that particular pest is not to be found.
Wireworms are a likely candidate or maybe keeled slugs? Not sure, we find rhizomes with holes in them, but have never seen a pest actually inside them. Usually it is the old roots which are affected and they are discarded in any case.

have split one in half and found a baby slug inside could this be the culprit if so how to treat as i have a dog and dont want to use pellets.

You could put down pellets but cover them with a stone or tile or something similar. As slugs like this kind of shelter they will find the pellets and expire out of the way of your dog.

will try that thanks all for your help.