Following my last post, I got some very useful feedback from those members who actually know what they're talking about in DCC. As a result the DMU was reprogrammed with one address for both chips and the lighting problem fixed quite painlessly. However, I was made aware how convoluted it can be programming a chip via a handset; the limited number of buttons on the LH90 for example make changing anything quite a chore. So with a little good homour intact, I dug deep and bought a Sprog whilst at York.
These wee beasties ain't cheap and they take some setting up (put the CD ROM in your computer which then tells you to download the software from the internet; what's all that about then?); however once up and running it was a revelation! Stick the chipped loco on the track and use the computer program to "read" all the settings; the program can even assess the kind of chip used. Store these on your PC. Change any setting; address, reverse mode, motor profile. All a simple and obvious click on the PC. There are some nice pages with graphs for acceleration profiles. I've not tried these yet, but as I become more proficient, I can see they will be useful. So all in all money well spent.