UCAS are clear that a university must not reject an applicant who has met their conditions.
That means that an Insurance can only let someone who has met their offer go into clearing AFTER their firm choice rejects them (a release into clearing can only happen if an applicant is UF with a university).
It *can* take days - in your case where you previously applied to York as one of your choices you're in a better position than otherwise.
IF you miss your firm grades
and
IF your firm reject you
and
IF your insurance either haven't decided or have you as UF on results day
(ie IF you end up committed to Manchester on results day) then once you have your results ring York and explain that you're waiting to be placed into Clearing - because you applied previously they'll be able to see your application and grades and might be able to offer some reassurance that they can hold a place.
In the mean time email Manchester's admissions department and reiterate to them ("following my phone call on Monday 15 August..."
that you no longer wish to take up a place with them and ask that IF you are rejected by notts then you would like Manchester to place you into Clearing ASAP (and preferably by 8am on Thursday 18th). That way even if it hasn't happened first thing on the 18th then you've sent written instructions explaining your decision.
The big problem is that one of UCAS key "Principles of admissions" is
"10. A provider must not approach an applicant who is holding a firm or an insurance place at a provider (other than their own)."
At the moment you're asking Manchester (at the busiest time of year) to go out of their way to help you out - when as far as their concerned they might be able to see that you've met your firm offer or been accepted with dropped grades. A LOT of universities don't look at insurance applicants until Tuesday/Wednesday - because otherwise they're doing far more work than needed confirming places for students who are accepted at their firm choice.
If anyone taking AS is reading this PLEASE don't wait this late to change your mind. Swapping/changing firm and insurance choices is EASY in July and a pain in August.