Personally I wouldn't go with a yellow splash back. Not because it wouldn't go but more that you already have enough of that accent colour around the room, over do it and it can look forced. There is a fine line between getting the balance right were it looks classy and going over board to the point it looks naff. Personally I would either go for a splash back that is colour matched to the walls or a smoked mirror. You are limited in some ways because it appears there wasn't any forward planning with regards to splash backs. Any splash backs would need to be confined to the width of the extractor because your curved wall units won't open otherwise. They would have to be packed off the wall if you wanted to go full width splash back. I would recommend a durable matt paint in the kitchen. Good quality ones will withstand washing without polishing up and showing tell tell marks where they've been washed. Little Greenes Intelligent Matt is perfect for this and is our choice of paint when not using any further splash backs above the granite upstands. I understand a colour matched glass splash back or smoked mirror may seem like the safe option and boring to some, but there is no point in being brave just for the sake of it. You've got plenty of other points of interest in the room with regards to colour without having to push it too far. Let the kitchen itself be a subtle canvas to show off other items rather than fight with them for attention would be my advice.

Yeah go up to the window ledge. I would say tiles are probably number one and mirror is becoming more and more popular. people tend to have quartz upstand if they're not having glass, mirror or tiles. I prefer it with out upstand on more modern kitchens as I like clean lines running through the kitchen.

Bronze Mirror will look very classy and will make the room feel more spacious. My no 1 in almost every kitchen I design. (Jo)

I have to confess it’s a nuisance when manufacturers produce worktops in one size and then upstands and splashbacks in another. If you are having the splashback all the way along the wall which will also go behind the hob you could have something different behind the hob which would give you the additional length you require. Usual hobs are 600 although some are 900mm. Perhaps have a contrast behind the hob? Premier Range have a webiste with some lovely options on. Bearing in mind you have chosen a neutral light grey you could successfully have anything with a hint of colour.