A UTR number is a Unique Taxpayer Reference number. It is 10 digits long and you are given one by HMRC when you register for self-employment.
Class 2 and 4.
A business expense is an expense incurred wholly and exclusively for the purposes of the business/trade.
It means that you have a prior year’s debt with HMRC (Normally more than 4 to 5 years ago) that has never been repaid. HMRC have stopped actively pursuing the debt, however if monies become available on your account (e.g. a refund from a tax return) it is highly likely that this will be taken to cover the debt.
Payments on account is a Tax payment made twice a year by self-employed people who are likely to have a Tax bill, in order to spread the cost of the year's income Tax owed. It is calculated by looking at your previous year's tax bill, and is due in two installments. The payment on account can be thought of as a way of paying off some of your tax bill in advance