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Found this on Spares Box. Thakns for the part number @TheApothecary.Major Bump. The links are dead in OPs post.
I used the RCA174MS (Trade price $30 - Retail $85 - the box contains 2 filters). You may be better off with HEPA filters off ebay that you can cut and shove in the void behind the mesh screen.
It fits in without modification however there's a 1cm gap on both sides, so it's not quite spot on.
Those plastic scrivets are Satan however, and I could only get them out via drilling the center and using a self tapping screw & pliers.
Also just wanted to say cheers for the help