Today water was seeping out of the light casing over the dashboard.
On investigation I discover the Ariel Seal was Leaking.
Is this a big and expensive job please?

And silicone sealer is available from the pound shops so its a cheap job!

As David says . Just seal it with silicone.
Should be simple and easy to do. If your lucky and the Aerial is above the interior light then just remove that to get to the fixing nut.
You might have to remove the sun visor too. This will allow the roof lining to drop more giving better access

Just loosen the nut on it the put some slicon sealer round a re- tighten.