Hi Guys. My son sent Me this and said what do I try now lol. He has rounded off an Allen key screw head. It’s tight for space so I guess 1) try and cut slot,or 2)vertical down facing grips,or3) JB weld a socket to it,or4) saw sensor off( it needs replacing )and hope I can get stud out.Thoughts please?

If you're dumping the map sensor, just break the plastic off (hammer and screwdriver) and that'll leave enough screw to hold with mole grips.

If you're keeping the sensor then I'd say cutting a slot is probably the safest idea.

Depending on the space available another possibility is to drill the head off then after the sensor is removed get mole grips on what remains of the screw.

I'd go with a small cold steel chisel and use a hammer to tap the screwhead and unscrew it.

If you’ve a set of Torx bits you may be able to find a size slightly larger than the hole in the screw head, so you can tap it in and get enough grip to loosen it that way. 
 

Or drill in to it if there’s enough access and use a tapered stud extractor to loosen it. 
 

Further option if you’ve got a dremel or similar and grind the head off to remove the sensor then mole grips on the remaining thread to get it out.