Household ammonia is (AFAICT) 5 to 10% ammonia in water, but it seems
hard to find in the UK. (I want to try using it to remove greasy film
from high surfaces in the kitchen.)

I have an old 2.5 litre bottle, about 2/3 full, of 25% ammonia that I
got from a chemical supplier about 20 years ago. It's been closed and
stored in a cool, dark place. Is there any reason why it wouldn't
still be usable (diluted appropriately)?

Thanks.