I have almost identical problem this one except that it's a shower tap...
How can I remove stuck anti-vandal rings in tap head?
The drama is that the shower won't turn off all the way as the washer needs to be replaced, however I can't replace it without first getting the tap head off.
Are there any other suggestions other than those in the other thread?
I've got a tap head spanner but it just slips when I try to use it.
Any help would be greatly appreciated.