I don't think you'll get a definitive answer on this.
My guess is that this happens to very few individuals who have specific skills that the regular forces don't have. When Telic was mobilised I met probably 40 odd former soldiers on the Reserve list that had been mobilised - again most of them had skills / trades that were desparately needed.
Many of them had been out of uniform for ten years or so. Did meet a few guys who on mobilisation did conversion to SA80 having never touched one in their careers, so it gives you an idea. Most of these guys "knew" that they could be in the frame - some though were completely unaware until the envolope arrived - how they laughed!
Of course, given the massive injection of cash and resources by the Treasury and MOD over the last few years, I cannot imagine that there are any shortages of skills / trades in Iraq or Afghanistan, I bet RTMC is nigh on shutdown.