Hi samwai,
If you are a UK tax resident you need to be completing self-assessment if your income is at a certain level, unless you are already doing this.
Check if you need to send a Self Assessment tax return
Tax on foreign income
You can call the NI helpline to discuss national insurance, they can advise you regarding class 3 voluntary contributions.
With a Personal tax account you can use the webchat services or call:
Number - 0300 200 3500
Overseas - +44 191 203 7010
Open - Monday to Friday: 8am to 6pm - Closed on Saturdays, Sundays and Bank Holidays.
Thank you.