Thanks for the link, I had a read-thru and as I thought, the purpose of the dyndns update is to update the host client with my routers IP as it changes.
It is this which I believe to not be occuring, as what I was experiencing was as my router ip address changes. My host name on the DNS site, is still (under the details section) showing the IP address it was originally synced with, when first created.
I must be missing something simple - so apologies if this is all a stupid question, but networking was never a strong point of mine..
and thanks for all your help.