Hi Simon, try some of these:
Is the screw just rusty, or is it stripped as well?
At the first sign of trouble, spray the screw with rust penetrant (Liquid Wrench and PB Blaster are two really good brands). Let the penetrant work for at least 15 minutes. Then spray it again and tap the screw head dead-on several times with a hammer. Then try the screwdriver again.
Squirt a dollop of valve grinding compound into the head of the screw. Then jam in the screwdriver and turn.
"Place a cutting wheel on a rotary tool. Rotary tools have detachable heads. You can swap out the heads with 1 of many different accessories. For cutting screws, you want a flat wheel designed to cut through metal. Fit it onto the rotary tool according to the manufacturer's instructions. Test the wheel by turning on the rotary tool. The wheel should spin freely at a consistent speed.
Cut a slot the size of your largest screwdriver into the screw head. Keep your biggest flat blade screwdriver nearby to use as a point of comparison. Tip the rotary tool so the edge of the wheel is over the screw head. Lower the tool down to begin cutting into the screw. Work slowly and carefully to widen the cut to the size you need.  An ideal slot creates a tight fit for the screwdriver so you can turn the screw with maximum force."
"Remove the screw with a screw extractor. The extractor should only be used if the screw head was damaged to the point where you couldn't fit the end of the screwdriver into the head of the screw. Drill into the screw head slowly with a cobalt metal drill bit. Insert the screw extractor into the drill. The screw extractor is essentially a drill bit that's machined in the opposite direction of a standard drill bit. Switch the drill to the reverse setting, and place the tip into the hole you drilled in the top of the screw. Squeeze the trigger to start the drill at a very slow speed, and allow the extractor and drill to pull the screw out of the object."
I found a couple ideas for you
Hi Simon, if there is no other way and it is unremovable, can you drill through it or cut the head off to make it flat?
If it is completely stripped, you may have to drill it out with a hand drill
You'll have to get a wrench and some muscle power to get it undone.
pour coca -cola on it ... it will for sure help
Spray it with WD 40 and let it sit about it 30 min then try to get it off
Spray WD-40 on and leave for 24 hours. Use screwdriver to tighten and then reverse to undo. If this doesn't work, use an angle grinder to take of the top of the screw. then drill out the rest of it. Hope that helps.
If all else has failed, I feel that pain, you might have to drill it out.
Hi Simon: You can do a couple of things. First of all, if possible, soak the screw with WD40 over night, put a screw driver on the screw and tap with a hammer and try to unscrew and screw it, back and forth. If that fails, try watching this video and see if there is something you might want to try. Just try to ignore the creepy guy:) https://www.youtube.com/watch?v=gjvgPJ0coo4 Good luck