Do I need to have a tachograph in a vehicle mounted MEWP?

The European law relating to the fitting and use of tachographs to record drivers' hours of work is contained in EC Council Regulation 3821/85.

All vehicles exceeding 3.5 tonnes maximum permissible weight coming within the scope of EC Council Regulation 3820/85 used for carrying goods (including the weight of any trailer drawn) must be fitted with an EC approved tachograph to record not only the driver's hours but, if a second driver is carried, his working periods also.

Exception to this rule:
A vehicle carrying goods, having a permissible maximum weight, not exceeding 7.5 tonnes and carrying material or equipment for the driver's use in the course of his work, within a 50 kilometre radius of the place where the vehicle is normally based and provided driving the vehicle is not the driver's main activity. This requirement also applies to journeys made to or through AETR member countries.

See: http://www.dft.gov.uk/vosa/repository/Quick%20guide%20to%20towing%20small%20trailers.pdf