Other TOC▸ 's are trying to go even further, for example, First TransPennine's fleet of 185's are fitted with a fuel monitoring system that will cause engine(s) to idle when not required and I believe that management have been trying to get a system in place where drivers would be rewarded if they save a certain amount of fuel.
Haing been told by one of their drivers what the fuel consumption figures are for a class 185 I can well understand why FTPex are rather keen to do the Eco Mode modfication and encourage their drivers not to cane the things harder than is neccessary to keep to schedule timings.
The Eco Mode shuts the third engine down rather than letting it idle, the units are designed so that the electrics on the dead car will crossfeed from the other cars in the set. Two engines on full load is more fuel efficient than three engines on part load.