Government spending/borrowing difference/ national debt increase disparity
There are many ways of calculating public sector finances.

One approach is similar to corporate accounting and the UK Treasury publishes Whole of Government Accounts.  

Those are different in construction to the Public Sector Finances statistics published by the Office for National Statistics, who follow international National Accounts methodology  

In respect of your question of what happens when these bonds mature and are in effect rolled over, does not lead to a difference in treatment: in both cases the repayment is not treated as government expenditure but of finance, and if the new bonds cover the same amount as the old bonds then in effect nothing changes and Government debt stays the same.

The translation from expenditure and revenue to borrowing and changes in debt is not simple.  The ONS produced an explanatory article. Separately the ONS published this diagram as part of a bulletin last year which illustrates some of the calculations they did for the 2015/16 financial year: