How do I get out a stuck sump nut that has rounded off 

How badly rounded?  

Grippy stud extractor socket would be the first thing I'd try.

Could potentially weld another nut onto it.

Not sure I'd want to try drilling it as they're pretty tough and would be messy.

Maybe just replace the whole sump if it's that badly rounded.

It's bad as I have got the stud extractor set and its not moving it at all 

If a new/good used sump is cheap enough, it's probably easiest just to swap another one in that case.

As the sump is full of oil, removing the sump will be messy, welding a nut on will be risky...sumps are thin so bashing a socket on will dent it badly. If you can fit an Irwin extractor socket with a  T bar, then hit the T bar with a lump hammer, that should shock the sump plug  to loosen. 

I will give that a go as it needs doing badly as the oil is well passed its sell by date

I would normally use Stillsons for a job like that with a length of hollow tubing over the end to increase the leverage if there is room.

If there is a risk of damaging the Sump it may be easier to buy an electric oil extractor instead.   

Blinking love Stillsons!  If you get them to grip, I completely concur.  My dad used to call them the universal graunching tool, but he was good with them.  If there’s enough meat on the plug bolt hex, is it worth cutting a slot across it like a flat blade screw to gain some extra torque (if you can get the hacksaw access)?  Then, get the Stillsons and a big flat blade screwdriver on it (or the like) and lean on both in order to double up the power you can twist it.  I’ve done this with very uncooperative pozidrive dome head screws and some mole grips a few times, but same principle.

I will give the stillsons a go I will give anything a try this will not bet me 

1 hour ago, Tizer said:

If there is a risk of damaging the Sump it may be easier to buy an electric oil extractor instead.   

I'll second that...  It's what I did with the rounded plug on my ST150... 😄

3 hours ago, nicam49 said:

As the sump is full of oil, removing the sump will be messy

The oil level should be below the top of the sump.  Use a (smooth operating!) trolley jack and wood block to hold it up while removing bolts, then slowly lower it and pour from one corner into the oil pan. 

I have done it once...but I didn't round that one, previous owner that time haha!! 

Alternatively, suck the oil out with an extractor first as suggested above.  

48 minutes ago, Marineboy said:

Blinking love Stillsons!  If you get them to grip, I completely concur.

I have built up a supply of several over the years, from about 3 inches long to about 2 feet. The big one is great for taking gland nuts off the top of struts on some cars.

8 minutes ago, TomsFocus said:

I'll second that...  It's what I did with the rounded plug on my ST150...

I bought one when Lidl had them but not used it yet, I gave my car its year one oil change the old fashioned way last year. 

If you've hammered one of these Irwin grip sockets on and it's still slipping with a lot of force behind it then you'll just end up stripping the threads and have to replace the whole sump anyway.

13 hours ago, Tizer said:

I bought one when Lidl had them but not used it yet, I gave my car its year one oil change the old fashioned way last year. 

I missed the LIDL ones so ended up buying an ebay one a couple of years ago.  The one I used on the ST was a manual hand pump which did the job, but the electric ones are fantastic!

 

Can't help wondering how  the OP managed to round off the plug nut.... I'm guessing it's probably 'cos every socket set comes with 12-point ones, and it's quite hard to source 6-point ones, though I only use 6-point impact sockets on my 19-year old car now.