[TestingWithEdd]

Use levels.fyi for general salaries, align Head of Development with the salary level you want to negotiate for, and use it as evidence for that rate.

Want £100k? Find the appropriate level on the site that has that rate and make an argument that head of development is that level. You can make this argument for basically any level, with that title, so this alone might work.

Probably argue that now you have line manager responsibilities alongside development requirements you feel the salary bump doesn’t adequately reflect the much greater workload, especially without additional equity.