Employment Lawyers Can Answer Your Employment Law Questions
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From what you say, it could appear that this is a shame redundancy. They have taken someone else on whilst putting you on furlough. Now telling you that you are redundant. It could very well be a case for unfair dismissal.
This type of document is common when someone wants to keep something quiet.
It is up to you if you sign or not. If the offer is very good and you think you would find a new job quickly.
If not, then consider employment tribunal.
How were you employed. Were you paye, where deductions for tax and NI were stopped at source each week, if so, they are responsible to settle tax and NI. They could be attempting to wriggle out of employers tax and NI contributions.