"They still get pupil premium but there is no statutory requirement for them to tell me what it's spent on (only Ofsted), and there is no statutory requirement for them to have a formal plan in place hat is shared with me?"
No statutory requirement, no. But good practice is to work in partnership with parents, and the evidence must show that the money has been used to raise that particular pupil's attainment. Most schools are willing to work with parents and, to be honest, you are likely to have far greater knowledge of how your child's early experiences could affect them than the school.
Start positive but firm, would be my advice. Key phrases such as As you are aware, I'm sure you know this already, and plenty of This is why I chose the school The excellent support you already provide etc.
Remember to keep them focused on using the PP to help your child achieve their potential- sometimes schools struggle to know what to do with a child for whom there are no obvious 'problems', and miss the fact that although they are comfortably achieving Bs in all subjects, they should be achieving A*s.
Using the money for staff training is a very effective use IME, and tends to be viewed in a positive light by schools who are sometimes reluctant to engage.
It may be a good idea to ask if the school's Designated Teacher for LAC can attend the meeting along with the SENCo, assuming that they are not the same person, as they should be able to offer a slightly different perspective.
Feel free to PM me if you want to discuss any specific issues you anticipate.