Vb 2015

My theory is that they just try to find the highest temperature possible- in the winter this will mean going as deep as possible, but when the weather becomes warmer in the spring, the soil will warm from above, and finding higher temperatures will mean moving upwards.

It's not difficult to model the soil temperature at different depths, given enough temperature data- I've done this based on air temperatures from Heathrow airport (airports publish loads of meteorological data) for 2008-2014, and this graph shows my predicted daily maximum temperatures at various depths for the whole of 2014:

I'd guess that most adders would be at 50cm-1m deep- the graph suggests that the temperature gradient at this depth would flip from hot below/cold above to hot above/cold below at about day 60 (beginning of march)- so that's when the snakes would start to move upwards. This is a very broad generalisation, though, and local microclimate would presumably have a big effect.

It's also interesting to look at the daily variation, here's one for Mar 15 2014:

again, the temperature gradient flips- this time it's at about 9am, so maybe this is the time that they'd be expected to emerge in the morning? 

Apologies for the long digressionary post, hope it's of some interest.

A switch in the temperature gradient acting as the trigger makes a lot of sense. I wonder if the next step could be to correlate the model to actual first emergence around the country?