Thank you all very much for your comments
I think there will be a complete 8 car rake of the GWR Toplights within the next few months, as there are so many nice variants of them to form a nice looking train.
"roamingstudio" The "snot" tiles are really only on sections of the carriages that do not have seats in-between them, so they can be built in a variety of different ways / techniques.
However, I have not yet put seats inside the toplight carriages, but I have placed minifigs alternate to each other at every other window which gives the impression of parallel seating and that worked well enough, although I preffer to leave the carriages empty for running at shows due to weight.
I think parallel seats may be a problem with these though,.....not due to the tiles, but due to the way the 2x4 curved slopes that form the tumblehome shape of the sides are built. I tend to build these with a few plates inside them so the carriages dont"squeeze" inwards when you pick them up (I like them to be sturdy), but I'm sure built differently that parallel seating would be possible, especially if using the new brackets that are due out