How can I remove stuck anti-vandal rings in tap head?

Question

Most of the taps in my new house are fitted with so-called "anti-vandal" screws in the tap heads, as shown in the image. These screw into the tap head to tighten them onto the stem.

I can't remove most of them because they are stuck. I suspect they've been overtightened and haven't been removed for years. I'm looking for suggestions as to how I can remove them so I can replace the tap washers. And yes, I know which way they need to be turned to tighten or loosen them ;-)

I've already tried:

• Using an anti-vandal tap head spanner (shown below), but the prongs bend/break, as the screws are quite stuck.
• Using a hair-dryer to heat up the ring before using the spanner.
• Adjusting the rings to tighten, then to loosen. I simply get no movement at all.
• Applying WD-40 to the rings. This hasn't worked either, perhaps because of their orientation; I can't immerse them.

I also have a pair of needle-nose pliers, but they're just a bit too big for the notches in the rings to get a good grip.

Any other suggestions?

UPDATES

1. UNECS's suggestion (below) was certainly a good one, but unfortunately it didn't work out.
2. I now have my hands on a small pair of 90° circlip pliers which I'll try later. Hopefully I don't break them (or anything else) in the process!

The anti-vandal tap head spanner:

Circlip pliers:

Answer 1

if you just need to replace the washers can you

1. turn off the water
2. open the tap fully
3. unscrew the cover flange at the bottom of the tap
4. fit a pair of mutigrips under the flange and undo the spindle from the breach

hence replacing the washers? Once you have the spindle out you will find it easier to get at the anti-vandle screw and undo it

Answer 2

Stuck screws around plumbing often result when water, and its dissolved minerals, wicks up into the threads and dries out, leaving rock-like deposits. On a faucet as you picture, wet hands will be used to turn off the valve, so you have a ready source of water above the thread even if the valve stem doesn't leak. Try stuffing a vinegar soaked cotton ball up next to the threads overnight, and see if that'll dissolve out some crud for you.