Reading between the lines (because of the lack of thorough reporting) I think there's probably a problem with his timeline, and filling up the 54 minutes between 10:30am (when he told police he had last seen Diane) and 11:24am when he called 999.
He went to Tesco to buy a few things and forgot his wallet (according to IS - but no proof of this trip).
Tesco in Royston is a 6 minute drive from the house.
Say 15 minutes to pick up the two items, head to the checkout and discover he had left his wallet at home. A trip totalling 27 minutes.
That leaves 27 minutes to account for before calling 999.
So he invented the CPR and standing outside the doctor's house for 5 minutes before making the phone call.
But the trouble is he told the call handler that he could go and fetch his neighbour so I don't think the jury is going to believe his time-filler. Especially since he also said on the 999 call -
999: Can you check her for me?
IS: Yeah.
999: Is she breathing?
IS: I don't think so, I've turned her to try to put her in the recovery position but I can't do it because she just flopped back. I think she's had a fit.
He doesn't say he's already attempted CPR for about 20 minutes. Plus he says he "just found her" . Plus he was counting too fast even for the two chest compressions every second - which makes it sound as if he was just counting very fast and not actually doing any compressions. Two compressions per second would be very fast, let alone going faster than that.
999: I'll tell you when to stop, sir, it's 600 times.
(IS continues counting from one to four).
999: Slow down sir, it's one, two, three, four.
[...]
999: You keep going, I'll tell you when it's 600 times, OK?
(IS continues counting from one to four).
999: Keep going, sir.
(IS continues counting from one to four, counting quicker as time goes by)
999: It's one, two, three, four.
IS: Sorry.