I'm afraid I have been unable to find any images of Darellella and Reynesella to compare. They are not listed in any of my books, on my copy of Paleobase or register any hits on the net. The keel, as you say, is smooth, so it cannot be Amaltheus. Still, the rock seems to be oolite as Andy has pointed out, and it looks so close to the link you kindly provided to Dorsetensia that I'm sure that be the beast in question, especially considering that the type of rock is very similar or identical to the Somerset beds.
Definately not Amaltheus so my moneys on Dorsetensia aswell
The ammonite fauna in the inferior oolite really is spectacular - I went to a dig at the Burton Bradstock caravan park a few years ago and it was amazing - didn't get much as most of what was there had already been spoken for but there were ammonites (mainly Parkinsonia) as big and bigger than dinner plates lying around together with huge Stephanoceras, Breydia, Leioceras and nautilus - dead ceph paradise! Did get one nice Parkinisonia (pic attached). The land owners were digging out hard standing for new caravans so this stuff was just being dumped as waste but the local collectors get wind of new exposures and get in there first. Still - it was nice to see.