No warning lights, so it's probably not a serious problem triggering full limp mode, does it die suddenly or gradually lose it's urge?

I'd get the fuel filter changed, could be something as simple as fuel starvation due to overdue filter change, when overtaking you're forcing through the most fuel at any one time.

Avoid general garages and main dealers, slip into a Diesel specialist, they'll read the codes and change that filter for you if they think its necessary.