I discovered this bunting this evening which I consider to be a hybrid Cirl Bunting X Yellowhammer. Head markings closer to Yellowhammer but stronger surrounding to earcoverts, lacking the black throat of Cirl. Also yellow central crown stripe like Yellowhammer. Upper breast with olive as Cirl Bunting but a bright rufous rump like Yellowhammer. The lower flanks were very heavily marked not really like either bunting! Cirl buntings songs are variable in speed and tone but this song was more like Yellowhammer without the 'eeez' at the end - some Cirl Buntings can be quite close to this. The call was closer to Cirl bunting but not quite right, it was more clipped and not so thin. The bird was quite close to the location of a strange very yellow headed bird in 2009, detailed in this link http://www.devonbirds.org/node/336 and in more detail in the Devon Bird Report for 2009.
As far as I know there are still no proven records of hybrid Cirl Bunting x Yellowhammer and even this one will have to go down as presumed but it must be as close as it comes!
Update: Following a phone call with Bruce Taggart this is a bird he saw in 2009 (he also considered it to be a probable hybrid) while carrying out Cirl Bunting work for the RSPB - even singing from the same tree!