Hi Rod!
15mm scale is 15mm to the foot, so like 16mm, but each foot of the real thing is represented by 15mm, 1mm less than in the scale you work to. It tends to be used to model 3' gauge prototypes (such as the Irish NG lines, the Southwold, and the IoM), which are usually modelled using 45mm track (3 x 15 = 45). It also means that the models work out just a little smaller all round than if 16mm scale was used - which can be useful if you're mixing and matching your prototypes, because much (but not al!) 3' stock is/was rather bigger than its 2' counterparts.
7/8ths scale uses 7/8ths of an inch to the foot, so the scale is bigger than 16mm scale - it enables 32mm track to represent 18" gauge prototypes, or 45mm track to represent 2' ones, and in both instances the bigger scale can be used to good effect, eg to add more detail or to enable smaller prototype locos to be modelled for easily. Hope that helps!
Great looking models Peter! I hope you've not completely abandoned scratchbuilding though, I love your unique creations?!
Andrew.