Thanks Gill and Nicola,
I contacted Purina about it and their explanation was that my local supermarkets aren't placing orders for it - perhaps customer demand is too low for it?
They said they are still producing it and have no intention of phasing it out.
In response to the other replies, I haven't found Winalot to make my dogs hyperactive; they seem to love it and if they're happy, I'm happy!
I see that it also bears the Royal Crest, so if it's good enough for the Corgis it's good enough for me!
Thanks for all your replies,
Liz