Also, to all of those that have fitted Sill Protectors, is it a difficult thing to do? I know it sounds silly, but when you have OCD even being off by a mm will drive me nuts. 
No it's very easy, a 20 min job. It's all in the prep and having a steady hand helps. You want to make certain the upper and lower plates line up perfectly

Firstly make sure the sills are spotlessly clean. I wiped down with IPA and dried off thoroughly. Then, before removing the backing tape strip, put the lower plate in place first, when you're happy with it, make a light pencil registration mark around the edge of the plate, then remove the backing strip and press down firmly to stick. Repeat with the upper ones.
The strong adhesive backing grips immeadiately so getting them right first time is crucial. The instructions suggest smearing the area first with a sudsy solution of washing up liquid to allow some movement whilst placing them, but this was more hindrance than help so I ditched this method it after the first one.