As well as posting it here I’ve added this pic to my previous post to keep it all together. This clearly illustrates what 999to5 said, with that straight edge and the plastic “rivets/poppers”
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So, discovered a few things today.
First is that the idea that prht drains don’t get blocked is a myth, cos mine were both blocked.
Second is that the ‘rivets’ appear to be a red herring.
The surface beneath them is smooth and they seem to be simply spacers to allow water to flow underneath the edges of, as well as through the grating.
The gratings seem to simply clip into place underneath the vertical plastic trim at the sides.
Thirdly, I can’t see how you could possibly get a trombone cleaning tool down the large hole under the grating without removing said grating
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Is it possible just to feed a hose pipe in the hole? 
Which would be my first port of call.
Even with a reducer like on a silicon tube would work perhaps.
(As you have it apart so to speak).
Save all that faffing about with trombone brushes etc.
I used a pint jug of hot sospy water on each side and a Lakeland plughole brush. Not sure the brush was entirely necessary but I was more interested in clearing the blockage before it tipped down with rain again than flaps which are probably already jammed anyway.
Worked a treat
I don’t think you need pressure and the holes are very easy to pour the water into
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Great pics, its hard to find any info on prht drains
Cheers
Dave
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Perfect pics!
Thanks.
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I’m glad I did take the trouble to take these pics or I wouldn’t have realised my drains were blocked, so win/win 
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