Can somebody id this for me please ?

Dear Andy,
From what I can see of your photo I am pretty sure this is Marianina rosea. We don't know very much about its biology apart from it being small, seldom more than 1 cm in length. The group of nudibranchs it is related to - the Dendronotina - are mostly cnidarian feeders, some, such as the tritoniids, feeding on soft corals, and others, such as the dotids and bornellids, feeding on hydroids. Perhaps your observation means that Marianina feeds on zoanthids. It would certainly be interesting to check.  I hope you will be willing to risk your zoanthid colony and keep the little nudibranch in your aquarium and see what it does. Even if it does eat the zoanthid, the size difference should ensure it doesn't do much damage - and you could be making a scientific discovery
Best wishes,
Bill Rudman