We later found this was because of De La Rue's production difficulties associated with the move from Dunstable to Gateshead, difficulties which saw production of the 78p, 88p and £1.88 stamps sub-contracted to Walsall Security Print. De La Rue printed the new Signed For stamps.
Now that the Presentation Packs are available, we find that that they contain stamps which are not the same as those printed by Walsall SP, which almost certainly means that they were printed by De La Rue. How can we tell, from single stamps? The Walsall stamps have a year code of M13L - those in the pack have a year code - like the Signed For stamps - of MA13.
Presentation pack (with guillotined edges)
We don't of course, know the printing dates for the De La Rue printings yet but will report them as soon as we do.
UPDATE 11 April
Thank you all for the contributions, we now know that Royal Mail's official FDCs have the De La Rue printings of all stamps. The presentation packs were available from Friday 5th April, which ought to make that the official first day of availability of the 78p, 88p and £1.88p despite the 27 March date that will be on those covers.
The stamps on the FDC received here are noticeably different to those in the pack. The text in the iridescent layer is noticeably 'blotchier', reminiscent of the late printings of the 2009/10 stamps. On most values the M of MA13 is almost a rectangle of dark, with very little definition.
The Walsall and Signed For stamps, of course, were available on 27 March, so those should be more collectable. Anybody doing double-dated covers?
UPDATE 12 April
It is now clear that some Tallents House subscribers for single stamps are receiving De La Rue (cut) examples whilst others are receiving Walsall examples. This may be because the DLR stock left over from producing FDCs and Presentation Packs is now all exhausted, or it may just be that the stock of singles is mixed and customers will get one or the other or a mixture. It seems that at this stage there are no sheets of De La Rue printing available.
UPDATE 9 May
De La Rue sheet printings are now appearing in post office branches. The £1.88 (and £1.28 MA13) were reported in mid-April, the 88p (and £2 MA13) at the end of the month.