Thank you so much for the replies - very interesting reading, and brought up points we hadn't considered.
As for flock health and bio-security - I hadn't thought of that, even though I know about isolation and checking for infections before mixing animals together. I have been careful so far in bringing in new sheep, and having them in a separate field for a month before mixing. Didn't think of this for rams - not sure why !
Our fencing is fine, so having a ram isn't an issue, but wasn't sure that having one for such small numbers was the way forward, but with info, perhaps it is the way to go. We don't want to be flooded with lambs, so we are being careful with numbers, and what can go to the freezer, and what will stay as breeding/lawnmowing.
Not really thought about cross breeding, as the Z and the W seem quite a size apart, and I would worry that a Z ram on W ladies, would cause size problems with lambs. We have plenty of land for them, so not looking at numbers, more for some lawnmowers, and some for the freezer, and are unlikely to breed them yearly.
Thank you again for your input, will head to the breedbook and breeder, and certainly consider a brought in young ram.