A good general purpose liquid feed such as Maxicrop, or Phostrogen or Miraclegro would be fine. If any of the plants are ericaceous, Rhododendrons, Camellias etc, then use Miracid.

Do we need Weed -suppressing membrane under Bark.?

I swear by Tomorite, Iwas advised to stop using miraclegro a few years back as it produces leaves and not flowers. I have never looked back with to tomorite

Thank you, Volunteer and Marion1. :) Don't those particular feeds have to be used every 7-14 days, though? I'm after one that needs applying only once or twice a year, and all I can find with that criterion are granular feeds that need forking in.

@Swanky - I guess it depends on each individual's situation as to whether it's needed or not. In mine, it was absolutely essential.

By its very nature, a liquid feed will only be present for a short time. The granular feeds release their goodness over weeks or months when the temp is right.