An extract from the article I linked to above:
"We have looked at the period from 1956 (when the published ONS dataset begins) to 2008, and taken the average current budget deficit or surplus over this period for Conservative and Labour governments respectively. There have of course been many ups and downs, for both Labour and Conservative governments, but long-run current budget deficits/surpluses appear to us to be one reasonable indicator for assessing “fiscal conservatism”, with lower average deficits being seen as more “conservative”.
On this indicator, in fact governments of both political parties show a current budget surplus, on average, over the long-run period. This is largely due to the fact that in the more stable (Bretton Woods) era before the 1970s and financial liberalisation, all governments almost always achieved a current surplus (but with high levels of public investment), while surpluses became much rarer in the more volatile post Bretton Woods era of financial deregulation.
Conservative governments have an average annual surplus of 0.3% of GDP over the 53 year period to 2008, while Labour governments have an average annual surplus of 1.1%.
(If we extend the period to 2015, i.e. to include the financial crisis and its aftermath, the gap would widen further as the Conservative-led coalition government saw large current deficits for 5 years, following very high peak-crisis deficits for 2 years under Labour. However, we feel it is more balanced to exclude the financial crisis data at this point).
We have also looked at the position from 1971 to 2008, and find that over this period, both Conservative and Labour governments have run on average a small current budget deficit.
In the case of Conservative governments, the average annual current deficit for this period is 0.6% of GDP, while for Labour governments over the same period, the average current budget deficit is 0.3% of GDP.
Therefore, looking at this one indicator in isolation, it is fair to say that Labour governments, over many years, have been somewhat more “fiscally conservative” than Conservative governments. We make this point as a matter of fact, not necessarily as our judgment of fiscal wisdom!"