Talking of strange poo's.....I buy up veg from the reduced section to steam to put into their food. The other day there was a large pack of grated carrot, so in it went, quite large quantities. Has had no noticiable effect on the big girls, but Mabs, the 7 month old youngster is doing bright orange poo's

Protien is not usually the problem, where it has come from is.
All proteins are not equal, most veg protiens are incomplete and are of no use to the dog, some meat protiens are more digestible than others, eg cold water fish such as Salmon are probably the most valuble for digestible protiens, while beef is not quite as good.
The problem with most dry foods firstly it the lack of meat content, 2nd poor quality bulking agents (wheat, corn, beet pulp, rice, oats, maize and so on) 3rd Protein source, as most manufacturers bump up the crude content by adding useless veg proteins.

As a matter of interest, does the lowering of protein also mean that the fat/oil content is lowered?

Also, is that across the board with ALL foods no matter WHAT the constituents?