Thanks for the help people. Before I left I tried to get the GMB Union I was with to get the redundancy paid to me after they told me I wouldn't get it, but they were useless.
They have said that I shouldn't have been paid any redundancy because I handed my notice in. But I handed my notice in because I was told I would still get my redundancy pay. It wasn't until at least 3 weeks I was told I wouldn't get it and only verbally. I have had letter saying anyone who left early would still get their payment, providing they are not needed within the company. Even though this is the case, I gave them about 6 weeks notice and they didn't let me know this until about 2 weeks before I was due to leave. Which I think is unacceptable.
Also, if a get paid something, I am to assume they have got it right, after all, it is money and they should be concerned BEFORE they pay you. But also, it was 3 weeks after they paid me before they contacted me regarding it.
I rang a solicitor today, but I only spoke to her PA, she was on lunch and I missed her return call. Hopefully I will be able to contact her again on Monday. I haven't responded to the letter they have sent me just yet, as I would like to know what I am likely to do before saying anything.
My current thinking is that a solicitor will say I should pay a compromise, so for example, offer to return half the amount, but I really need to sit down with someone and show them the letters I have had etc.