Peugeot 306 HDI 5 Door

[MY95]

I think someone needs to re-test it haha, I do think 30% is a bit too much, maybe 10% ish but there's definitely a difference.
If I added an extra 30% onto that 52 mpg I would have got 68 mpg, which would have been pretty epic
Well yeah you will use more fuel when its cold as its warming up, compared to staying at around the same temperature without the need for the bits needed to help warm the car up.
Then again on your DV6, the different speeds will be likely to affect your mpg much less than it will on a 150k+ HDI at different speeds/revs, due to the age of the engine and the technologies etc.