It's the fact you've established it's Cretaceous, otherwise my initial guess would've been Psiloceras psilonotum, which alas is from the Hettangien...
A further clue might be found in the systematic account of the Ammonoidea in the 1966 Geol. Soc. Memoire on the Geology of the country around Canterbury and Folkestone
"Except for the Desmoceratid Beudanticeras, which is common in the mammillatum Zone and occurs frequently in the early beds of the Upper Gault, the "Leiostraca", or smooth ammonites are known only by a few chance finds of Hypophylloceras, Tetragonites, Pictetia, Desmoceras, Puzosia and Uhligella. These ammonites are thought to have preferred open waters and to have had their European centre of dispersal in the Mediterranean region."
The problem obviously being, that your picture is of a much more slender shell than any of the quoted species above...
Is your fossil much eroded? The posted picture is somewhat vague, so I can't make out very specific markings...