[Mcfly]

The overflow bottle catches and refills coolant in the cooling system. When the engine runs hot,
more coolant is pushed into the overflow bottle, as the coolant is under pressure, and is forced
out of the cooling system to maintain pressure (dictated by the pressure cap)

So.... the coolant level will fluctuate in the overflow bottle. IF your consistently needing to add coolant,
you may have a problem as Teampryor stated. A brand new car MIGHT still be burping air out
of the system that the plant didn't get out, which means your fluid level would drop until the air
is gone. Keep your eye on it.

Having your coolant system checked will do no harm, and cover your ass, as you'll have documented
the coolant loss, and attempted to resolve the issue with Mama Ford, (As Teampryor recommended)
it's your best option.

 

[Mcfly]

Hi

I haven't checked it since i got back on saturday (it was 1" below max line then) and tbh i don't think i should bother again as they can't find anything wrong - i was thinking of letting it run out of coolant and letting the engine go bang as this will be the only way they believe me when i say it's loosing coolant

I've never had to top coolant up on a new car - the only time i had to was on an Audi which had a leaking radiator - not happy having to keep checking on this Fiesta

For those of us @nal retentive folk, regularly checking fluid levels is a way of life..... heh heh. I personally wouldn't let the engine grenade, but I WOULD show
up at the Service Manager's desk once the bottle is really low, show them the last time you were in (receipt), and let them do the math, AND top off your coolant.
Repeat as necessary... Persistence pays, and after the third or fourth time, they're gonna get it, and you'll save $$$ on anti-freeze.