From what I understand, the Civil Service pays for academic achievements based on relevance. If you manage to find a position in another stat board / ministry asking for post-grad qualifications (could be in engineering or not), then yes, your salary would increase substantially.

But if your current position doesn't require a PhD, then chances are you're on the same progression rate as other graduates who have no PhDs. It's all down to performance, and to a large extent, how well your bosses like you (huh).

I've heard of several graduates who attained Masters while working in the Civil Service then resigned because it simply didn't help them get promoted (one of the reasons is also that the Civil Service doesn't recognize your Masters if you attain it part-time while working inside the Civil Service).