I work in HR. I also haven't read all the responses.
I work for a company where you have to declare an additional job, although I have never disciplined someone for it. If that is what the contract says, then yes they could being a disicplinary but actually this is more a breach of contract than a disciplinary matter.
However, there are many things that are not right here and not providing an investigation pack that the company will be relying on during the process is a big no-no. Have they interviewed you at any point? Was there any reference to an investigation pack in the letter? If not, then go back to them and query this robustly because this is a requirement
Also, check the letter sent to you and ensure they have provided you the right to be accompanied in the meeting - failure to provide this is also a problem.
After the meeting, make sure when you are provided with the outcome you are given right to appeal the decision, as thats breach of the ACAS Code of Practice.
This has got tribunal written all over it and because you have a protected characteristic, you don't need 2 years service to bring the claim. Any HR support they have in place will be having a meltdown right about now and if they aren't, they should be.
Also, one last thing, but double check whats happened with your tax code. You can have 2 jobs but your tax free personal allowance can only be applied to one job and any second income is taxed at basic rate. Theoretically therefore you should apply the tax code to the job with the highest wage, which i am presuming is your current role not the work you are doing for your OH.