Hi any ideas where the best next place to look where my car could be losing brake fluid from? I've checked the wheels, master cylinder etc, nothing on drive over night, losing about a cup a week at the moment!

 

If you’ve already checked all hard and soft lines, the next place I’d check is the ABS unit. It’s on the right side of the engine bay, can’t miss it.

 

No loss of pedal pressure? That is a significant amount and if you have no issues with braking and don't see it spraying out under pressure I would suspect reservoir or ABS unit but should be easy to find that much fluid, it will remove the paint off anything it is coating have you removed all the undertrays and looked for a wet spot?

 

this, it should be pretty obvious. If you can’t spot it get it in to a shop that’s quite a lot and sometimes leaks on the braking system get larger from the pressure so you could end up rupturing it and having no brakes at all

 

If you winter drive I bet the rear lines under the liner are toast. I had to chop and replace mine.

 

It is leaving a closed system so it is going somewhere and if it isn't leaving spots on the ground then it is leaving spots on one of the under trays somewhere.

 

Check the vacuum line going to your brake booster, if the master cylinder rear seal is leaking itll leak into the booster and the vacuum from the engine will suck it in an burn it through the engine. That's alot of fluid to go missing without a trace and that's literally the first place I would look. I've seen this happen on various cars.

 

I was going to recommend what you mentioned to the OP but you beat me to it.
In all cases that's sounds like a lot of fluid disappearing and needs a very close check before something serious happens