Two years ago the Environment Agency started to release LiDAR data (Light Detection and Ranging) for the UK.  This is an airborne mapping technique where a laser measures the distance between the aircraft and the ground.  Most of the UK is now mapped and the data can be downloaded for free.  Height measurements are recorded as a grid with a resolution of 2m, 1m, 50cm or 25cm.

Free software such as QGIS can be used to plot the data.  This is a 25cm plot of the Merstham Rockshaw Road/M25/A23 junction in Surrey:

This could be rather useful for mapping lines of shakeholes or sinks.

So far about 70% of Wales has been mapped and the data is available from Natural Resources Wales.