I would have said the spring would be the best time to use by working it in between the plants but it could be a mulch between the existing plants. It will only act to help moisture retention so if you wanted to give your beds a lift, then something like 6X would be my choice. I would do it sooner rather than later as it can cause lots of soft, leafy growth later in the season. If it's just to improve the flowering, then a sprinkle of sulphate of potash will do the trick.

Agree with Jimmy!

I've got a different take on this to Jimmy and Moon grower. Yes, order the compost, enough to go over all the borders, preferably 1-2 inches deep. It shouldn't be spread if the soil is bone dry, but I'm guessing its not, currently, with this changeable weather we're having.

To clarify, what this will do is improve the soil over the next few months to a year rather than fertilise your plants directly. But improving the soil means its more fertile, there's increased bio diversity, and plants growing in it will find it easier to seek out what they need. That old adage,feed the soil and the plants will feed themselves is absolutely true.

For an immediate boost, then a general fertilizer such as 6x or just growmore would do the trick- but I don't recommend it at this time of year if your borders are planted with shrubs and perennials, its too late, as we're a month past the longest day already, unless your part of the country is very mild or near the coast.

If you can add more composted stuff in spring as well, your soil will be in much better condition next year, and so will your plants.