We recently received a C3 envelope folded and taped to be almost square, cancelled with Manchester's 'Large Flat' postmarking machine.
We first saw this a few years ago on a bulk mailing but have seen little reported about it since, and no extension to other areas. The place and date portion is partly printed on adhesive tape, so it doesn't show, but there is no doubt that this is Manchester Mail Centre in November 2016. Rather than any seasonal message it has the Postcode slogan.
Look out for unusual postmarks on your Christmas mail, and let us know what you find. We'll be reporting more on slogan postmarks very soon.
GERMANT ~ Germany ATM FDC - Germany 2017 ATM FDC, posted on January 3, 2017 from Bonn to Taipei with arrival postmarked on January 19, 2017.