Thanks all, I made it!
Despite the SW being rocked by a fairly major storm early in the week, I took a gamble and drove up to Bodmin on Wednesday, during a break in the weather. Sorry, Mr H, I should have called you but we weren't sure the weather would hold. As it happens, we were lucky!
I approached from the west and parked at the end of the track, near Camperdown Farm (various signs at this point state that there is no further "vehicular access", and that the track that continues through to Fernacre Farm is "private").
I was surprised at how close to the farm the Louden Hill circle is, and the Frenacre circle was just a short distance beyond that. I'd have loved to explore more, but had to get back to the car and Mikki.