If I recall correctly the A350 doesn't use a trim tank but manipulates the leading and trailing edge flaps to manage the trim. Saves weight on the plumbing etc but adds complexity in other areas.
The trim fuel is clever and works well in my experience but it will be interesting to see how the system works on the 350.
Found this after a Google, so give it the same amount of credibility that any Google info should be given
The A350 is the first Airbus wide body aircraft to not have the trim tank in the tail that uses fuel to improve cruise performance by adjusting the center of gravity (though I should note that early A300 and A310 variants also lacked a trim tank). On the A350, a variable camber wing that adjusts minute deflections in the flaps accomplishes the same functions without the complexity of a trim tank system.