Best fertiliser for soil in spring?

Hello! I'd like to give my soil a boost and I was wondering what the best general ( maybe slow-release ) fertiliser there was on the market, preferably something I can get at my local garden centre or Homebase? Any recommendations would be most appreciated!
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I had to stop using it, for this reason and now use Growmore.
for the fag ends of the aristocracy.
It's best to feed soil organisms as much as plants so I never use manufactured stuff like Growmore as it is a by-product of the oil industry and is expensive to produce, in energy terms. However, if I had fox problems I'd probably have to think again.
If a particular flowering or fruiting plants needs a tonic, I use liquid tomato food.