http://www.caa.co.uk/docs/33/SafetyNotice201104.pdf
The CAA have issued new guidance:
"As a result of insufficient assurance for the accuracy of the vertical modelling, Volcanic Ash TDAs have previously extended from surface level to the upper limit of the modelled area of high ash concentration. Therefore, although procedures for the overflight of Volcanic Ash TDAs are in place, there has not previously been the ability for operations beneath areas of high ash concentration.
With immediate effect, Volcanic Ash TDAs will be promulgated by NOTAM with an associated upper and lower vertical extent. The publication of this information has been made possible due to improvements that have been made by the Met Office during 2011 to the dispersion model that predicts the movement of volcanic ash in the atmosphere. These included improvements to the way in which the ash concentrations in vertical layers are calculated. However, in order to mitigate an area of residual modelling uncertainty, the vertical extent of the Volcanic Ash TDA will include a lower vertical buffer zone beneath the level of the high contamination area depicted on Volcanic Ash Concentration Charts. This policy has been agreed on the basis of specialist scientific advice and accounts for the meteorological and eruption strength uncertainties that are inherent in this forecast."