As you have probably guessed MDLMat will only let you edit the materials that are already in a model. It won`t let you create new ones.
A new material needs a whole new material section added to define the material details and a new texture entry in the texture section if it isn`t using one of the existing ones. Then every part that uses the new material needs to be altered so it will use the new material instead of the standard one after dark

This is easy if you are the developer working with the source but difficult or impossible if you are trying to hack the finished model.

Sorry but if it hasn`t got night textures already built in there is not much you can do

Thanks for that, Martin - that's much as I expected. I'll contact my forum mate, who made the models, and see if I can progress from there.
And thanks for replying so quickly - do you ever go to bed? Don't answer that!
Cheers - Dai.