I can definitely understand that seeing a vehicle repeatedly slow down and then speed up, might introduce doubts over whether to continue to pass it more than once. But in the OP's scenario, the ambulance passed him then stopped and so he then "overtook" it. Just seems a bit odd to have any question marks around that at all.I kind of get it - as I mentioned above I did a few miles awhile back leapfrogging an ambulance with its blues on but no siren (except a couple of times when they came up quickly behind me) which kept slowing right down or stopping and in some situations it was like "do I overtake or not?!?". This was around 2am so little in the way of other traffic.
Guess the reality maybe just doesn't come through very well on a forum post.