Thanks all for the comments, I'll try and just use one post.
We booked through ***** and they asked no questions about mobility. I now notice that when you try and book there is a tick box about mobility issues so I'm guessing I'm not the only one. I also notice now if you try and book direct through P&O there are now tick boxes about evacuation chairs when you do a dummy booking. It will refuse a booking where there isn't one.
I made final payment at the time I booked, this was a requirement of ***** at it was 1 May for a 17 August cruise. No there was no mention of an evac chair (people told me to check it was booked on, which is the only reason I queried it).
I have no idea who I am dealing with, I don't seem to be offered anything but the option to change the name on the cruise (I don't see why this helps my parents) and they can move the cruise to within 12 months. I tried to move it to a Northern Europe cruise on the 16th October, no evac chairs were available.
I send a form via the online cruise personaliser bit, I recall last year I e-mailed something, this year it was done online and a PDF was generated. But I didn't e-mail it, it was submitted automatically. They did confirm they had received a mobility questionnaire.
My wife and kids will still go in August and my parents are vaguely happy to move their cruise (I'll still go with them, so I get an additional cruise). They did Norway last year, so happy to do something a bit different.
I'm not immune to media reports about people getting turned away from cruises, but the P&O spokesperson accuses these people of not filling in a medical needs questionnaire, which we did.
Appreciate all the responses and further requests for clarity