Thanks for the replies.
As I said in my original post I had left it a bit late to ask,so had left before the replies came.
Anyway,we did manage it in the end courtesy of Richards Bros.
We caught an early bus to St Davids,the Celtic Coaster to St Justinian`s in time to get the 10 A.M boat to the Island.
By getting the 4P.M. boat back we managed to catch the last bus back to Newport.
A bit tight on times but do-able .
We paid £18 for a weekly explorer ticket, which was extremely good value as we probably used more than that in our first two days.
Thanks again and I will try to remember to allow more time for replies next time.