How old is "old" and is it still quite dry?

It is about 3years old & still dry.

I would have erred on the side of caution and said "Chuck it", but my OH says, if it's dry it probably won't have degenerated much, if at all, and that you don't have anything to lose by trying it.
If you do decide to throw it away, make sure you take it to the proper department at your local tip rather than just dumping it in with everything else.