No payment dismissed ill health

Recently been dismissed due to ill health on 19th Jan. In the meeting I was told I would be paid payment of notice of lieu plus holidays for 11 years of service totalling around £3500. I never received payment or letter but did get p45 this Tuesday. I rang them everyday since chasing payment and my termination letter. I was promised I would receive payment today. After not getting a payment I rang them to chase it and now I have been told that they overpaid my sick pay in the beginning and so now I'm owed nothing. So I will not be getting anything from them. This is their fault. Their mess up. Should I still receive this payment that they told me I would be getting? 
I never got any pay slips from them apart from the first 2 months. I was off for 16 months. Also never got my termination letter and right to appeal until I chased it and got it via email yesterday. Promised payment and now told they overpaid me so getting nothing. Is this right? 
I have got in touch with ACAS and they have recommended to fill out an early conciliation form, which I am. 
Have they got the rights to just not pay me my money for my years of service plus holidays? Due to their mess up? After I was told I would receive this payment during my meeting when I told I was being dismissed. Also stated in my termination letter I received via email. 
Obviously I'm very frustrated, ill and have no idea whether they can do this to me or not. I feel it is unfair. And to only find this out after chasing them after not receiving promised payment. Any help would be appreciated. Please help. 
Thanks 
zoe