Sorry Yak - I do respect your advice but it turns out I'm more inquisitive than I thought.
I've been messing about trying to make a socket to fit this big nut but I really really can't weld for toffee so this attempt failed. May be this idea is good for someone who can weld? Probably not though I can't see it holding 220Nm - the plan was to use a big spanner on the outer sides of the bent bit of metal...
So plan B has been to visit the CNC boys and get a new socket made.
This has almost worked out well. There was a bit of a problem with first and third angle projection and the meaning of dotted lines. Here's a picture of the bit I asked them to make. Unfortunately they ended up making something with two male ends instead of a female 8 sided socket with a male 6 sided end.
If anyone wants to use this drawing please feel free to do so at your own risk - I'm not a professional draughtsman and there was some confusion when I showed it to the (Dutch) CNC chaps here.
I'll post a picture of the modified part when the CNC chaps finish their work on it. It will end up being a bit shorter than the 60mm one I designed - it will probably be closer to 40mm and not as meaty - I hope it will be good enough for 200Nm. We'll have to wait and see.
(Happy days!)