[omits]

When is the best time to buy this for next year? I see B&Q have it @ 2.5Ltr for 13.55 and 1Ltr Liquid Seaweed for 9. Perhaps around December?

[New shoot]

I usually find December a bit hit and miss for gardening stuff.  It tends to get shoved out of the way for the seasonal decorations in most DIY sheds and garden centres.

My local garden centres quite often have pre-season offers on fertilisers very early in the year when they have packed away the tinsel and are setting the garden sundries section back up.  Last year I got some very good deals and stashed enough for the whole season.  Other options would be the shops like B&M, Home Bargains, or The Range.  They only offer gardening stuff over the spring and summer, so sometimes you can be lucky this time of year when they are clearing summer stuff away. In the spring when they are setting it back up again, they sometimes have offers as well.  It would be a bit later than the garden centre deals - say March/April rather than January/February.

[Subversive_plot]

If by seaweed, you mean seaweed extract products, They usually have a limited shelf life, beyond which some of the active ingredients (natural plant growth hormones) may no longer be useful. On the other hand, the trace elements and nutrient components will still be useful.

The packaging should have an "expiry date".  Make sure that your purchase is dated beyond when you expect to use the product, especially if you want the benefits of the plant hormones.  If you need the product to have active plant hormones in July, a product that expires in May will disappoint you.