Question

The roof seam all around my Galaxy static caravan is leaking badly, what can I use for a permenant repair

Answer

The problem that you have is that hese seams will expand and contract with heat and will also be affected by the wind which will cause movement at the seams. The panels and edging strips are bedded on a non-setting mastic which should allow for this movement. If this breaks down you either need to re-bed the panels on a mastic such as IDL or clean the joints and apply a good fillet of Sikaflex 512 caravan sealer. This sealer allows some flexibility at the seams would hopefully cure your problem.
Do not use any kind of silicone sealer as this will shrink in time your problems will re-appear.