The copyright and acknowledgement requirements for the CEH data can be found in the Educational User Licence.
Original copyright of the underlying topographic base maps lay with the Ordnance Survey. This has since expired with Copyright of the land use data belonging to Stamp. On the death of Stamp, copyright passed to his assistant, Audrey Clark.
If you incorporate the data within your own work the following acknowledgement should be cited:
© L. Dudley Stamp/Geographical Publications Ltd, Audrey N. Clark, Environment Agency/DEFRA and Great Britain Historical GIS