Franke Ascona SP3308 Tap Valve Adaptor

If you have a dripping Franke Ascona Tap, or maybe one of their other taps fitted with a SP3308 1/4 turn ceramic valve, you may find it difficult to remove the old valve. This is because the valve doesn't have a conventional hex head, it's rounded with just 2 flat edges 12mm apart, like in the picture. The valve is also inset into the tap body, so it's very difficult to get an open ended spanner onto it.

I tried removing the valve but the spanner just slipped off. I also tried mole grips, adj