Franke tap handle screw seized up

i have a Franke Zurich kitchen tap and need to replace the ceramic disc valves. The instructions are straightforward: water off (at service valve - done); remove the H or C screw cover (done); remove the screw [.. and slide off the handle.] - PROBLEM.
Not the usual problem of the handle being stuck - I cannot undo the screw. It has both crosspoint and straight slots (see photo) and I have used both types of (manual) screwdriver but cannot get enough torque to loosen it.
I have tried the following without success:-
Turning clockwise to break the seal;
Tapping the head of the screwdriver (gently) to break the seal;
Squirting some white vinegar around the screwhead;
Squirting WD40 around the screwhead.
Any suggestions about what I should try next?
The tap is only two years old and Franke promptly sent me replacement valves (5-year guarantee.) I expected to replace the valves easily, as I have done previously on a much older Franke tap.

1. Try pouring boiling water over the handle to expand the female thread.
2. It's probably a Phillips cross head, rather than Pozidrive. They are very slightly different. If you have a No. 2 Phillips, try that.
3. Failing the above, drill it out. Not easy to do in such a way as to be able to remove remaining thread or re-tap. They are usually M4 brass screws.

Drill the head of the screw off. If you're replacing the valves there won't be a need to re-tap. As oldbuffer says be sure to replace the screw with a brass one.

Thank you both. I'll go and get a No.2 Phillips driver, and then try the boiling water and using that, before resorting to drilling out. I'll report back

Job done - didn't even have to try the boiling water. I have a small & flimsy multi-head ratchet screwdriver and thought I'd check that Phillips No.2 was the right size in the screw. Not only did it fit, to my surprise it was able to undo the screw where the Pozi and flat drivers failed.
Thanks again to oldbuffer and Elkato531, and I hope that this thread may help others in the future.