We have a 74 Islander 30 Mk II, she's our pride and joy. Not sure how much benefit this to you but my water tank is as well directly under the forward V berth, the only access I have to it is by a plastic screw on circular cover that's is situated directly in the centre of the V berth approx. 1/2 foot towards the bow from the drop in cushion.
When I fill up with fresh water I run a garden hose through the forward hatch, pull the cushions aside and unscrew the cap. This cap is about the size of 6'' pizza. As for my holding tank, it's located under the starboard dinette seat that's closest to the bow. The holding tank is fairly small but works well for what we need.
Can't wait to get out on the water again.