Another how to access a tap washer question?

The top section of our bathroom taps is a dome with 3 small knobs around the side to allow you to turn it. It sits snugly on the base. There is no visible screw holding it in place and there are no inserts of any kind which could cover a screw. How do we remove it?
We can only think of 2 options - to pull the cover vertically upwards or to turn the tap fully on (having turned the water off first!) and then hopefully keep turning to unscrew the cover. Either way, it could be expensive if we choose the wrong one!

D&S
August 2007