Mini One Coolant Loss

Coolant needing topping up every other day.  Had
pressure test done, checked the thermostat
housing and no gunge in the oil cap.

On driving the car with a full coolant reservoir after
a while you can hear "bubbling" and when
checking, the reservoir is drained.

Had it in the garage 3 times already.

It's a Mini One (not sure if that's the same as the
Cooper)

Any suggestions ??