I would imagine that the index can be unscrewed using the serrated edging.
I'd be very surprised if it doesn't unscrew.
Fantastic it does unscrew
Don't bother, you can't.i will take it apart just in case you can replace it
Just have a look, might be a washer might not. How long have you been working on this tap now.Fantastic it does unscrew took some doing as it was well and truly stuck, I see the screw now to undo it, but now you say you cant replace the washer so its not going to be a cheap 5 minute job, i have loads of washers, i will take it apart just in case you can replace it, i never knew that some tap washers cant be replaced.
If you go for a decent brand name- Grohe for example, you just phone them and they often send one out FOC.
Same with Bristan, although the taps aren't as well made.