The facts:
Told I was being made redundant on 6th August.
Told to go home as there would be no work for me but to be available in case they needed me.
Letter received on 7th August stating that redundancy will be paid end of 31st August (it was) and that any other pay and accrued holiday will be paid on 28th September and I would also get my P45 at that point. My last day of work would be 24th September.
On 31st August I received a letter saying so long and good luck and enclosed was also my P45 and a payslip. The payslip detailed my redundancy payment plus PILON. The finish date on my P45 says 7th August.
I dropped into the office to see HR ON Wednesday to find out why I'd received my P45 early and my PILON and they told me that I wasn't in fact on gardening leave until 24th September and I was in fact free to find employment from 8th August. They also said that they were not told I was on gardening leave.
Email clarification received yesterday:
"I can confirm you are free to start new employment. Your leave date is the 7th of August in our system, which makes you a leaver from the 8th of August. As your leave date has changed after the letter was issued you will not receive any further payment from us."

That states that my leave date changed. At no point was I advised of this.

So, I've been applying for jobs since I heard I was being made redundant but was doing so assuming that I wouldn't be available to start until after 24th September.
I believe that they have effectively prevented me from being able to work since 8th August.
I did speak to ACAS this morning and they said I should write to the company with my grievance including asking for recompense for potential lost earnings. I was offered a part-time job but had to turn it down as I was told I was on gardening leave.

Has anyone else been in this situation or have any advice about my next course of action? I'm very angry about this simply because I could have been earning money for the last month!

TIA