http://www.teachat.com/viewtopic.php?t=4508 (also about powdered sencha) and now I realise that I don't understand the boundaries between konacha and powdered sencha at all.
That's bad enough, but, looking for something else (the London Takashimaya doesn't seem to have its own website) I just came across this web page from the New York Takashimaya (a very well known and highly respected Japanese department store) which blurs konacha, powdered sencha AND matcha
http://www.takashimaya-ny.com/product_d ... 24&cat=254
(kona means 'powder' in Japanese)
Teasweetie - there's very little information in English about powdered sencha. And there seems to be lots of different kinds of this tea for all kinds of purposes. To be on the safe side I would personally seal it up as best as I can (the main thing is to protect it from smells and moisture) and put it in the refrigerator. Matcha goes off very noticeably after a few months, I'm not sure if your powdered sencha will too. If you have a lot of it, you may want to try other uses for it. For instance, you could try dissolving a spoon or two of it in a very small amount of water and blending it with vanilla ice-cream. If that works out, you might like to use it to flavour other sweets and cakes.