What's it in, and have you got any torx bits already?

I've had success in the past with using a centre punch to bang the security pin out and then using a normal torx bit, although obviously that's not idea if it's in something fragile/the torx screw isn't made from finest chinesium. 

Just get pretty much any old cheap set with the right bit (looks to be T10 from your pic), the amount of force you'll be putting on it and the amount of use it'll get (I'm assuming this is pretty much a one off need?) you don't need to worry about bits rounding off or anything. 

 

Some of the cheap tools are made with really poor quality steel that's got no strength. In particular drill bits that will last for maybe half a hole before they go dull or just overheat and snap in two.

It's got a lifetime guarantee that one though. :C

Defective by design.

The free ones that come bundled with things like mobile phone batteries are always entertaining. I have had some that are OK, but some are so bad you've got to despair of the human race wasting the Earth's resources to make them. T10 is a bit big for a mobile phone though.

They're in my assbox, so I do need the screws to remain functional.

The existing torx screwdriver I've got wouldn't fit even without the middle bit.

If I can spend £25-30 for a set capable of handling any encounterable screws, that'd be great - so long as they're not weak pound-shop-quality crap.

What do I need to unscrew this?

Step 1: Get Ireland to unite, team up with Scotland.
Step 2: Get them to take over the rest of the UK.
Step 3: Celebrate the United Republic of Ireland and Britain with tea and flapjacks.

:)

Can we please mandate that we should all have a southern Irish accent?

Because it's the best accent ever. (nod)

They can be serviceable (or even good) in a pinch: while on vacation last year, the mirror on my film slr got stuck*, an ongoing issue that can be remediated by unscrewing the bottom plate and oiling gears (as I learned from a web tutorial, except my body is an older and more simple model than shown).

I needed a very small phillips head screwdriver, so I bought a set of miniature screwdrivers at ye local dollarstore, and had the camera back in action in <1 hour.

*blocking the viewfinder. I took several more, perfectly good shots using the point-and-shoot method, to ascertain this would not dislodge the stuck mirror.

I was anticipating a video hosted by some young Olympian hunk.
 

I was unavailable.

Do you mean southern as in not-Ulster, or more cardinally (as in Cork, maybe Kerry, but not Dublin or Galway)?