The Brew Cabin

I already have a riding arena 40m x 20m, with no drainage, was thinking
about porous pipes. One down the middle (40m length) then from this, one
every 5-10m to each side, all at a depth of 1M. Will this be of any use?
Been looking at the drainage for fields and Gardens pages. Would I need some
where for the water to go? I live in the fens and the riding surface stays
wet, odd puddle, for around 1-2 days after heavy rain. Any help/advice would
be great.

There's a few other threads on this subject, and I'd suggest you work your way through them to assimilate the various nuggets of advice.

As I mentioned in the other Horsey post today, there's a school of thought (ie, me and a membrane specialist) that a drainage composite would be a better construction than a permeable pipework system and/or a non-woven membrane system.

Whichever construction you choose, you will definitely need somewhere to outfall the collected water. Whether that's a main sewer, a soakaway or a ditch, depends on the site itself. As your arena is already in place, constructing using a drainage composite is impractical, so maybe a permeable pipe is your best option, but you still need somewhere to outfall.