Jules is you man with the gills of a fish.
other than that have you tried pushing a hose with a spray end back through the bottomdrain pipework, this sometimes breaks it up enough to clear any blockages.
a) Yep - thought of Jules but would rather not drag him 2 hours each way just to get uncomfortable (but happy to have him if he is able to fit it in)
b) Thought about hoses but think I have to do it from the pond end as I have a 4 foot vertical to a 90 degree bend then long pipe run to the drain - am sure I would not get the hose past the 90 degree bend and suspect that the blockage is in the bottom drain at the mouth of the pipe as that is the narrowest point.
Next time (not that I want there to be a next time) - Ithink I will put an air nipple into each of the long bottom drain runs so I can purge with compressed air if similar happens in the future.
now theres an idea.
gently push an airline down the bottom drain pipework. (it will go round the bend) till it gets as far as you can go, then connect it to an air pump/compressor and pull the pipe back sl;owly. you should clear lots of crud and hopefully the dreaded blanket weed.
even attaching the airline to your net handle and poking it down the bottom drain might clear it.
There is a guy in Waterloville think he's an ex=royal navy diver with all the gear does this sort of thing for members of the south hants koi club .
if your still in need let me know and I'll get you a phone number.
Am a member of South Hants and that was where I was gonna call - but hopefully I won't need to due to the genius we know as Vippymini....
I have left my 4 foot vertical pipe with 100lts/min airline acting as an airlift to give it a bit more suck - hopefully by the time I get back tonight it will have done the job.
If you get really stuck I may be able to help but am quite a way away!
And would have to get my cylinders filled.....
On the subject of divers, there are actually loads around, especially nearer the coast that would probably be very happy to help.
They would probably take a camera in with them