It begs the question how good was the original training if they then need to take another course. It sounds like the training company are chancers and just trying to sell you another course. Your engineers just need to be competent to carry out the work at height. Make sure you carry out a risk assessment etc for the work at height to make sure you have all the bases covered, and maybe have a practice session at ground level, making sure it is fitted correctly, finding suitable anchor points, remaining clipped on when moving around etc.

For the harness specific training were the company aiming at rescue training also?? I have had attended training courses involving harnesses and they only tell you how to put them on, nothing about rescue from a harness, perhaps thats where the company is coming from.