Will they be taking parcels which the post(wo)man couldn't leave as no-one in at the adress? Or will we have to traipse on umpteen buses to the sorting office which I understand will be in the wilds of the Heatons?
You already can get your missed collection delivered to your local post office. For a small fee.
Many post offices also accept parcels ordered from online retailers to be collected from a post office for free. Marple currently (as far as I know) does not offer this.
Details of both at
https://www.postoffice.co.uk/mail/collection-servicesI expect their plans to take parcels are the latter. I sincerely doubt they will do a deal to allow you to pick up undelivered mail for free given they already have other options to deal with this (post office pickup, redelivery etc ). Especially as this is not a service they generally offer in urban areas.
I could be wrong but why would they bother?