The only one to Arillas from Sidari is the return from Corfu Town via Sidari at about 4 or 5 pm. I can't remember the exact time, but it's about then. Someone will doubtless put up the exact answer. If you are in Sidari, you can get a bus to Arillas then, but not one back.
There are more from Sidari to San Stefanos, but you would have to make your way to Arillas from there. A long walk.... Or simply stay in San Stef. If it's a naturist beach you want, Maggie has answered your question. In our "walking days" Jen and me have used the very unofficial North end of San Stef. Turn right as you face the sea and walk till you see others....
Hope this helps
Phil
