Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
Is the £18,000 income as an employee?
For 2016/17, you will pay £174.60 in Class 4 NIC and £145.60 in Class 2 NIC on self-employment profits of £10,000. Take a look here for the rates and thresholds.
If the £18,000 is income as an employee, you will pay £1,192.32 in Class 1 employee NIC.
I hope this helps but let me know if you have any further questions.
In that case, you won't pay NIC on rental income and trust income.
The first £11,000 of your total income in 2016/17 will be tax free and you will pay tax at 20% on £17,000, assuming the rental income is net of deductible expenses.
On £6,000 employee income, you would pay no NIC.
Your tax figure of £4,200 and the net income figure are each correct.
If any of the trust income is dividends, then a different rate of tax will apply. Look here for more information.
Look under the heading "If you're employed" here. You pay £12% employee NIC on earnings between £155 and £827 per week.
UK trust income tax is charged to the trust on the trust's income at different rates depending on the type of trust and the type of income as you can see here. In your hands, the income is taxed differently. Interest and rentla income will be taxed at 20%. Dividends in excess of £5,000 will be taxed at 7.5%.