Weatherproofing plywood

You only need the silicon on the edges and between the sheets and join when it's wet, that rainproof stuff should do for the surfaces.
Bear in mind this is a bit of a bodge because you want it done quick.

 

Any advice guys on what type of screw the op should've using here, particular reference to countersink variety .......

 

A good exterior one.

 

I would go for a 30 x 5 double countersunk, drill and countersink the ply first

 

Ply will delaminate pretty quick if left outside.To stop ply delaminating, seal the edges with epoxy resin, you want it pretty thin to let it soak in. Expensive stuff though. Have you thought about using Ronseal wood hardener instead?

Shame you didn't go for proper marine ply as it's guaranteed for 10 years, submerged in salt water.
Again, very expensive stuff but the dogs......
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