First up in the remedial process was to carefully rip out the old platform top:
You can also see the disused platform in the foreground which is also experiencing an "upgrade"! As well as the plywood, I also removed the topmost layer of sleepering to allow enough depth for the replacement top.
Now I'm not an avid fan of plasticard either, but for planking etc. I don't think you can go wrong with the Evergreen sheets. Machined rather than moulded, they have a definition lacking in most of their peers. A base of 30thou was overlaid with some plank effect sheet. This had to be done in panels due to the curve on the platform and individual edge pieces were also used:
Once dry, the unit was offered up and trimmed slightly to effect a close fit with the base edge and the join with the brick built platform:
Whilst I tried to avoid it, the retaining wall had to be removed to allow satisfactory fitting. I'll use that as a reason to have a better stab at the wall.
Before gluing down with a smear of evostick, I painted the unit with enamels and weathered with powders. A little more blending will be needed as the other elements of the platform are placed, but so far I feel it is a big improvement over the original: