I experienced a case where the person being made redundant saw 'her' job being advertised externally during his redundancy consultation and within a few weeks someone ( a friend of a senior mager) was appointed. But the new job was said to be at a lower salary, reported to a different person and the job description was written in such a way that the employer could say the job was sufficiently different. But sure enough a few months after the redundancy the new person's role was expanded and the salary was increased substantially.
In your case there may be a genuine change in market conditions or the role may be regarded as sufficiently different. Having seen an employee shafted I wish you luck with any claim , but maybe you should consider whether you would really want to work for them again, and whether any compensation would be worth more than your severance pay.