Thank you for your detailed reply.
Here is my recommendation to you
Personal tax (HMRC)
If your circumstances were to change and you were to file a tax return in the future, then you need to register for self assessment and you will be given a unique tax reference number (10 digits). You would use this number when you communicate with HMRC.
You can register for self assessment online and the link is here
Although, if HMRC ask you to complete a tax return you must do so. This will normally be to make sure you are paying the right tax and getting the right allowances.
If you don’t already complete a tax return form, you’ll need to do so if you receive any of the following:
Income from savings and investments of £10,000 or more.
Income from untaxed savings and investments of £2,500 or more.
Income from property (before deducting allowable expenses) of £10,000 or more.
You receive income from overseas
Income from the estate of a deceased person on which tax is still due.
If you or your partner receive child benefit and your income is over £50,000.
You state you had problems finding your tax reference number when asked for by NI Contributions Section of HMRC. The payslip would normally have employers PAYE reference number.
It would be advantageous for you to write to NI Contributions Section at Newcastle address and request they provide you with a statement of your NI contributions to date for pension purposes and also update your records to reflect your current address and your status.
You have outlined your plans for the future. If you were to get involved in the running of a LTD company, you may be deemed as an employee. You will have to consider your status at that point. If necessary, register for self assessment at that point.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.