Lost my job after three weeks after HR admitted to making a mistake during the hiring process
As per the title, I'm looking to see whether I have any leg to stand on legally here. This is a throwaway account; I'll try to be vague in order not to cause myself issues. I am in England.
To be brief (and apologies, it's still pretty long), I applied for a new job with an employer I had previously worked for and left as part of a resignation agreement. The scheme maintained that any future job must not be "like-for-like" within five years, and the employer would be under no obligation to consider an application within this timeframe. Fair enough.
I have in the year-and-a-half since leaving changed the direction of my career, and I thought I would chance an application in a new department with this same employer. My approach was that if they tell me it's a no, then I've lost nothing, and if they agree that it's okay I've landed a perfect job for a new career. I mentioned in the application about my previous work, made it to interview, and then made absolutely sure to mention the circumstances in which I left the employer previously. I asked that they double check that this position did not contravene any terms and conditions, and they confirmed they'd look into it. I specifically asked them not to offer me the position if there was any uncertainty.
I subsequently received an offer, which I happily accepted. At this point, I considered the matter to be settled: I had hidden nothing, they had considered me, and decided to offer me employment. I assumed the terms of the agreement had been reviewed.
I handed in my month notice at work, and subsequently started work for this employer. After three weeks of work, I was summoned to an HR meeting with very little preamble. I didn't really know what it was about - I was only told it related to my resignation, but when I asked if it was anything serious, I was told that it would all be cleared up in the meeting. I assumed I must have to sign something, or similar. To my surprise, I was informed that HR had never actually looked into the agreement, and my employment was immediately terminated. They admitted a mistake was made in no pre-employment checks being made, but that they'd subsequently reviewed the job spec and they think it's too similar to my prior employment. I was quite stunned. They agreed they'd pay me for the month I had worked, plus a month in lieu. I was then walked off-premises.
My union has advised me that due to the short period of employment, I don't have a claim that can be taken to tribunal. They said I'm probably not getting a better deal than the 1-month-in-lieu. (I'd like my job back, not compensation.) They did advise I might have a stronger case under a civil jurisdiction, due to negligence on the part of the employer - but that "the prospects of a substantive Employment Tribunal claim presently appear limited due to insufficient qualifying service and the absence of an automatically unfair reason for dismissal."
I have very little legal knowledge, but I have two contentions. First: I don't believe the job specs are especially similar, except in the broadest possible reading. I'm not sure how to contest this, or if I even can. I was hoping my union might be able to help there. I'm not going to post the job specs here, but advice on who would be best to speak to would be appreciated.
Second, since I am now in a significantly worse position than if they had simply not offered me the job (having to explain a gap in my employment and find work urgently), am I in a position to take any legal action, e.g. with relation to negligence? How would one go about this?