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Thanks Wiltshire Builders.

Ive tried Rain-ex (not the pure one - the one with the in-built cleaner) it doesn't work but I might try the pure stuff now.
I know that rain-ex is not meant to be used...

I have tried various things which I can't remember the names of. The only thing I have found that actually works and lets water bead off is SDoc 100 plastic polish.