These are NOT scarce as these stamps are from rolls of 500 & 1000 which have been sold by Royal Mail Tallents House.
The 2nd class MA10 coil has been found in kiloware, so if you have the time, it could pay careful searching! See the comment on the original post.
A few more have now been found in kiloware, and these images provided:
And this image shows two different mailings from the STWC.
Stuart has also found an example in kiloware on a postcard mailing, so probably not from the Sunday Times Wine Club although possibly from the same direct mail house.
Total number found now known to be 10 - but what are they worth to collectors?