IRD Filler Nut

Folks,
Just tried to remove the IRD Oil filler plug and looks like I’ve damaged the head. I changed the IRD oil about 40K ago and it was tight when I took it off then but looks like the guy who changed my clutch last year didn’t intend it to come off again when he topped up the IRD oil level. Also, looks like signs of some kind of thread locking / gasket agent around the filler plug / nut.
Has anyone had any issues with the IRD filler plug or can anyone suggest what I can do (if anything) without taking the IRD out and spending £s. I’ve used a socket that is intended to grip the flats of the nut head rather than the corners but with no luck.
I’ve no evidence that the oil level is low (no strange noises etc) but nevertheless I would like to check it.
Thanks in advance.
Chris

six sided impact socket and long bar, if all else fails a hammer and chisel, suggest you purchase a new one to put back first though, i am sure hippo put pictures up of one he removed with hammer and chisel

as far as I can remember its a 19mm,try heating and whacking an 18mm socket onto the nut.I had the same problem last year completely made **** if the nut.
I was thinking of giving up 'till I heated a 17mm socket till it was red hot ,then I got an old 9" extension onto it and whacked the socket onto the nut using my trusty lump hammer,then I cooled it with squirt of water.
Got the bar onto it and it came of easily,the socket had an incredible hold.

Thanks for the comments chaps. I'm away on hols next week but will sort it out on my return. In the meantime I'll order a new filler plug and washer. God knows why it was put on that tight after all it's suppost to be removed at every service?????