Base for decking

[Spennomort]

Hi - I am looking to install some decking to replace old unsightly concrete and part of my lawn. I have removed the concrete and turf for the intended area but am finding that the ground beneath is a little soft and not draining particularly well.
I never had any problems with the previous concrete so am a little concerned about how to achieve a firm base for the decking.
My plan is to put down a layer of pea gravel as a base and also mount the decking on paving slabs (400mm square). However, is there anything else anyone can recommend to give my decking a solid foundation.
Thanks in anticipation