Discrimination- first offer

Hi, I've taken my previous employer to early conciliation (for context I was only there 4 months and I believe they sacked me because of me having fibromyalgia and arthritis in my back, I have a credible witness who is willing to do a statement for tribunal and attend if needed, she resigned over them saying they were going to sack me and that they weren't going to make reasonable adjustments) Acas have emailed me today and my previous employer have made an offer of £2500 but say they do not believe discrimination was done. They've also said that this is their full and final offer. Could someone advise on how they think I should proceed, my witness is willing to stand up in tribunal, I've got screenshots of our conversations and her saying they told her they were going to sack me (this came from the chief financial officer) because I'd use my fibromyalgia as an excuse to have time off and I'd actually never even taken a day off!