Ian,
What type of site are you looking for? Whenever we visit Machynlleth to see family, we stay at Furnace Farm CL (page 457 Caravan Club Directory), roughly half way between Machynlleth and Aberystwyth. They have elec hook-ups, a small toilet and shower which is basic but adequate and wonderful views of southern end of Snowdonia mountain range towards Cader Idris,