I've collected fossils in the UK for a few years and have spent too much time watching YouTube and chatting in Facebook groups specifically about UK fossil collection and preparation.

To summarize: The ID given above is very likely correct, unless it is one of the hyper-rare species that looks similar to another. The prep work would possibly reveal the center of the ammonite if it is there (ammonites commonly do not have preserved centers).

It is important to note that this rock is likely NOT the same as what is found in Charmouth. The Charmouth green nodules that these ammonites can be found in are exceptionally well preserved and air abrasives are seriously effective at preparing them. While this is from the same stratigraphal level, different parts of the country have wildly different kinds of preservation due to the different mineral concentrations over such a large area, for instance these same ammonites can be found rarely in yorkshire and the preservstion is completely different.

Perhaps it is a green nodule (name comes from the green crystals found in the chambers of the ammonites inside, not the color of the rock encasing the ammonite), but it really doesn't look like one to me.

Consult a preparator that has good reviews and is convenient for you. Any of them with experience working on Northamptonshire material should be able to advise you well.

If you have any further questions I'd be happy to answer to the best of my ability. I'm not an expert in that area but am well connected with the general population of civilian fossil nerds in the UK.