I'll spell it out for you.
Debt is what we currently owe. It's fairly regularly measured in telephone numbers, or as a %age of GDP.
Deficit is what we're going to be short of this year (ie, going to owe). Again, it's expressed as phone numbers of a %age of GDP.
This year's deficit goes into next year's debt figures. As far as I can see, the sums work out such that you can add the %ages together for a very rough estimate (obviously GDP changes over time). But if we start with a debt of 70% of GDP, run a 10% deficit, it's only matter of a couple of years before we're approaching 100% debt/GDP.
It's no good saying "Oh, we have a relatively small national debt, so let's carry on as per" when doing so means running a deficit of approx 12%. That 12% goes into your debt figure at the end of the year.
Anyway, don't take my word for it. See what our competitor countries have to say:
http://news.bbc.co.uk/1/hi/8569418.stm