Here's something nicely collectable that reminds me of the old days! De La Rue were unable to meet a deadline for these values to be available at Post Offices so they subcontracted the printing to their rival, Walsall who provided the initial counter stock for 2 April. De La Rue eventually got their act together (they were moving premises) and provided the stamps that were issued in presentation packs and orders that the Philatelic Bureau had delayed. I also understand that De La Rue stamps are now getting to Post Offices too.
Now, why Walsall didn't simply continue and supply the Philatelic Bureau and maintain counter stocks until some future change I don't know. Maybe it's more convenient if there is just the one supplier of main type of definitive stock. Anyway, that should make Walsall items worth looking out for as they will be comparatively scarce.
They're also easy to distinguish, thanks to another instance of DLR slightly slipping up. Walsall stamps have M13L as the date code, following the traditional layout so far in this respect. DLR, for some reason best known to the guy setting up the plate at the time, used MA13! Great!
The Walsall prints (the lower row in this illustration) have a slightly lighter head and different security slits too.