Been comparing the stats for London and Paris. Basically I ran the first half in London a minute quicker than Paris and held on to that in the 2nd half. Although 2nd half of London and Paris were roughly the same times, in London I maintained good pace until the last few miles then slipped a few secs per mile. Different in Paris, I lost pace initially after half way but then blasted the last few miles.
In summary I think I gave myself a better chance in London- built a buffer and then held on, rather than playing catch up and failing.