Overview of the side rail.	Removed the end caps and the plastic insert.	Didn't expect this! The rail is rivetted on, expected screws, there must be no wooden batten behind.
Used a large drill bit to take the head off the rivets.	After removing the rivet heads, pulled one end and it just came away, easier than expected.	Does not seem to be a great deal on sealant behind the rail.
Not much sealant, but there is no damp in this area.	Pulled out the rivets with a pair of pliers, which left holes in the skin.	Area thoroughly cleaned with a rag and white spirit, when dry filled holes in the skin with Sikaflex 512 Caravan.
After cleaning the rail used W4, 19mm strip mastic applied to the centre of the rail.	Applied Sikaflex 512 Caravan to channels top and bottom, it's an adhesive/sealant so no need for screws.	Used two end cap screws to locate and pressed into place, then more sealant top and bottom, finished with a finger.
Removed end screws, then refitted with end caps and new trim, pre-drilled to aid fitting.	The end screws go into battens under the skin.	Looking good, now I know what's involved when I tackle the others.

Note; To get a better finished result, I will use masking tape next time. (See: Re-sealing Fridge Vents)

Materials and Cost: Sikaflex 512 Caravan, adhesive/sealant £6.50, W4 19mm mastic £5.25 for a 5m roll, Plastic trim Burgundy 45p per metre.