right, i've just spent the best part of the day being diverted from the switchboard of the belfast harbour commissioner and the belfast harbour police regarding this matter and this is what i have found out.
firstly the belfast harbour commissioners' office don't seem to know their bum from their elbow on this matter as i got bumped about the switchboard from one dept to another before being re directed to a very helpful constable harkins at the control room in the belfast harbour police HQ. he was kind enough to spend a bit of time tracking down the by law in question which is by law 42.
the gist of this, is that you cannot fish anywhere within the belfast harbour comissions' duristiction which is a bloody big area as it starts from the north foreshore at carrickfergus in a direct line across the lough to grey's point at bangor

the wording of the by law (which is an old one) is that you cannot fish with rod, line or net whithin this limit from any jetty, wharf, pier or bank.
so my OCD kicked in and i asked "what about below the high tide line"
he laughed and said "well there is no mention of that, or from the shore either in the by law".
i explained that we fish and dig down there and have never had an issue in the past as we've seen the belfast harbour police drive past us many times and even gave us a friendly wave as they went by

.
constable harkins then went on to explain that it's more to do with a health and safety issue from within the harbour estate and so long as we did'nt venture any furthur into the harbour past kinnegar we would be fine as this by law is an old antiquated by law that you could put into the same catagory as being arrested by the police for not carrying out your long bow pratice for any male over the age of 14 in the uk
more to that there's never been a case of anyone being fined at greys point, helens bay, crawfordsburn, seahill, cultra, holywood, loughside, hazelbank, whiteabby or whitehouse.
i asked him had he ever known of a £30 fine being issued and could'nt recall an incident of it ever happening and said he would only ever imagine that happening if the offenders in question had been up to no good at the time.
but this still left me puzzled a bit and he said that in future if you're going to go down fishing just phone ahead to the belfast harbour police HQ on 02890553000 to let them know you are only fishing and there should be no problem provided you don't block the road with your motor or start making a nuisance of yourself by lighting fires or littering
so hopefully this puts everyone minds at rest and finally gives some resoloution to the matter