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There are some that are definitely more difficult as slightly older seeds than others. Parsnips are almost always germinating sparsely if at all a year on, peppers and onions are getting a bit tricky after two or three years. You should get 4 years from a fresh packet of lettuce and brassica seeds. Beans and peas also squashes and pumpkins are 5 or 6 years, tomatoes up to ten years. The problem is that you do not know how old the seed was already when it was packed. And that is the rub.
Thanks for all the helpful comments. I think I'll presow as many of the older seeds as I can rather than sowing later/direct into the plot. All the advice much appreciated