I spent ages trying to work this out and ended up cheating by watching a video of some bloke riding the course in 2013 on YouTube. I was looking for the road sign to confirm location. I sort of ruled out all my possibles by the signpost not being there but I've just realised it may have been installed since the video. It (signpost) wasn't on the entry to Greeba Castle in 2013 but the crest looks about right.