sideshow is quite correct about the 0.4: Assuming that is about 10-11 hours, you would be entitled to roughly an hour of PPA.   But ....

Since the STPCD refers only to 'tinetabled teaching time', which it defines as the 'period of time in the school timetable during which the teacher has been assigned by the head teacher in the school time-table to teach pupils'  

I can't see any reason why you actually wouldn't be entitled to full PPA on all of your timetable.

I'm not sure how much planning, reporting and marking you have for any of your work but your head might be able to argue that, since you do less P, P and A for all of the teaching you do, perhaps you might have less PPA and in the interests of goodwill or good sense (definitely not the meeting ground for a lot of discussions on P&C in a lot of schools) you might agree to a reduction in your overall PPA.

Try negotiating around this from the position that you are entitled to full PPA under the letter of the law, make it clear you are prepared to do the school a favour by making concessions on your entitlement and then work out something which seems reasonable to you. If your head continues with the original assertion, seek union support from your regional office to fight it.