When calculating the average tax rate over the last three years is this done on the basis of P60s or do I also need to take into consideration any extra income e.g dividends, rental income etc..?
This is an area of tax law and I think you should seek the services of someone qualified. It's like a legal problem. Would you try to sort it yourself?
Your average tax rate for TSR is your average income tax rate. You pay Income tax on dividends and rental income so they would be included
Not sure about DIRT ,this would bring your average rate down as it is at 20%, it is income tax at the standard rate,would be surprised if the revenue give this