Same here OP. I've moved jobs within the NHS but under a different directorate. A recent pay restructure has resulted in me getting paid less than my colleagues ( a few £k) and the same amount as a ward housekeeper.
You wouldn't believe what my new role involves compared to that of others.
Seems mightily unfair but the way I look at it is, I get vast experience which will enable me to go on to a better paid job later. My CV will look fab!
Why would they mention what other staff are paid at interview?
They've evaluated the job at a lower rate than historically some people are on. They are the anomaly, not you. This isn't about you being on the wrong salary, it's them on the wrong salary. You are not entitled to know what your colleagues are being paid, and in this case asking them to not tell new staff isn't underhand, it's because it's of course demotivating to find something like that out.
If it was an equal pay issue, and they asked all the blokes to keep quiet that they were being paid more than the women, then of course that would be outrageous. But when reevaluating and regrading a job as lower than previously, a line has to be drawn somewhere.
"I would have thought they would mention they had recently undergone a pay restructure and the salary has been amended within the last 6 months"
Why would they mention that at interview. I can see an argument that it might be an idea to mention it when you started work, but why to candidates for the job?
Look, the salary for the post has gone down. They had to draw a line somewhere so that everyone recruited after x date would be on the new salary. That line was just before you joined. No doubt you'd prefer the line was drawn after you started, and no doubt the person starting next would prefer it to be after him/her, but it is where it is. I know it feels unfair but what else should they have done?
If they have absolutely no option, companies are doing that, sure. But that isn't the case here.
Clearly you don't have much of an idea about how difficult it is to cut salaries, both legally and in terms of relations with employees. There is no way they are going to go through all that just to save the feelings of one new member of staff, even if they could do so without getting into legal trouble.
I think one of your earlier posts - "not on a bad salary even without the £3K" says it all. It sound very much as if they were overpaying, and have restructured down to a market rate for the job. The other staff shouldn't really have told you what the pay was, and I think your best bet now is to try and put this out of your mind and buckle down to the new job. If you really find that you can't do that, your only option is to leave I'm afraid.