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In case anyone is interested, here is an update. The item is still at a sorting centre "awaiting a flight", we are now into the fourth day when it has not moved at all. Royal Mail apologise but point out that it is not a guaranteed date of delivery type of service. Normal airmail would have got there by now and the blurb on the Airsure service (encouraging you to look upon it as a worthwhile spend at 9 times the cost of normal airmail) says it normally gets there up to a day ahead of normal mail. Royal Mail then go on to say (in a telephone conversation) that they take no responsibility for delays - if it is lost they will refund postage. Definition of loss is the failure to deliver within 20 days of posting.
It seems to me that unless you want proof of delivery (which Airsure allegedly provides) then there is no justification for paying the money for Airsure - it is at least potentially much slower than normal mail and in my case (and how many more) definitely so.
Shedman, you are correct and I knew of the saynoto site (and now have an alternative number) but I wanted to see how much this nonsense can cost - I too deeply object to all sorts of agencies/institutions/authorities barricading themselves behind a money-making wall. There was a period when (apart frfom 999/112) the only way to contact our local police was through an 0845 number (I think it was, not an 0870 which is even more expensive).
Spudqueen, you are of course correct on your explanation, but to me tracking is no such thing if all you are told is "It is somewhere between your post office and the destination". That is what appears online but by telephone (see above implications for most people) you can actually find out precisely where it is (supposedly, although they say it may have moved on), not that that is much help when it sits there day after day.