Rejuvenating spent compost?

[Tiercel]

As one who does not like to waste anything if I can help it. Could do with some help. I have a lot of spent compost, what would be the best fertiliser to add to it to revive it? I am thinking along the lines of FBB or even osmacote? As they are a slow release fertilisers.
Any thoughts please all advice welcomed. :)

[Tiercel]

[agapanthus]

I'm gonna use all my spent compost as a mulch for my veggies at the lottie :)