No no no no no, 100% do NOT soak this, it will completely destroy it. I have collected and prepared many of these Apporhaid gastropods from Folkestone successfully and this is the method I use.
1) After collection, if the clay is wet, first fill any major cracks with superglue then put it in a plastic bag (doesn't need to be completely sealed) and leave it to dry over a period of a few weeks. This will allow it to dry without the clay cracking and the shell lifting off. 2) Prepare the shell with a round-ended scalpel and then use a small paintbrush dipped in water to gently clean any remaining clay off of the shell. 3) Coat the entire shell in Paraloid b72, use quite a strong solution, maybe 20-30%. I know some people say use only 5-10% to reduce shine but to be honest that's so weak that it will not stop the shell flaking and deforming, you need to harden it completely. For fragile areas you can apply thin superglue to the surface of the shell. There is probably no need to coat the matrix in anything if all the cracks have been filled but if it is very flaky in the past I have first coated it in paraloid and then scraped off the surface to restore the natural look. As I stated previously you absolutely cannot soak these blocks. Solid pyritic and phosphatic fossils from Folkestone should be soaked to remove the salt, but the most you can do with these clay blocks is preserve them. The effect of salt decay is generally minimal on gault blocks, it's not like chalk fossils where echinoids can literally explode off the surface of the chalk in 10 years if not desalinated properly. The majority of gastropods in museum collections from the Victorian era are still fine if they haven't succumbed to pyrite decay by then, and they were certainly not soaked. That's a nice specimen of Pseudanchura carinata by the way, shame about the area of missing shell As an example here's a specimen from Bed VII prepared in this way recently
If you don't look, you won't find.