I have noticed a large area of white powdery like substance on my rockery and all over my composter which is near to it

could it be a load of baby spiders???

Sounds like mealy bug offspring or type of aphid which will suck the sap from your plant and pass on virus or even kill your plant. Zap them pronto with dithane or buy a cheap spray pesticide gun for a couple of quid from wilkos.

The powerdy substance is probably powerdy mildew(fungal) and aphid thrive in damp wet conditions

Sounds like baby spiders to me too.

Count the legs of the bugs, 8 and they are spiders, 10 and they are bugs!

6 legs and they are an insect but as to which one? Bugs are a class of insect.
10 legs puts them ino the crustacean group, woodlice etc
2 body sections spiders
3 body sections insects again we need a photo really to give the best answer,

what about ants?

welcome to GoY too.

Bugs have mouth parts which suck, that is the deffinition of a bug.

yes i agree but many people use the term bug to mean any critter or microbe. bugs are one of the many groups in the insect class. [they are hemiptera] so they will have 6 legs.