Pay reduction and redundancy/impact on notice pay

Superclarkkent

Yet another 'what am I entitled to' question coming up!

I have been informed that my position is being made redundant and the process will be finalised next week.  A bit of background:

1. Been employed for 9+ years, so notice period is 9 weeks and I am entitled to 9 weeks redundancy pay (capped at £538 per week).  I should add that 9 weeks is my contractual notice, and I think this ties in with statutory as well.
2. I have been on furlough since April when we were told that our pay was being reduced to 80%.  Employer claimed the £2500 furlough and then topped up the difference to make my monthly salary 80% of what it was previously.
3. We received a letter in April notifying us of the above, and had to respond saying that while on furlough we would not carry out any work for the company.
4. We didn't sign or confirm explicitly that we accepted the pay cut.
5. In August we were told we were taking another 5% pay cut.

My understanding is that my notice period should be paid at my original salary.  Is there any way my employer can legally get around this and only pay it on the 80% salary, or the 75% salary?  They may not try this, but it wouldn't surprise me if they did either so I want to be sure of my rights.  My concern is that they can somehow claim that the current amount they are paying me can be claimed to be my 'non-furlough' salary as they are still topping me up. 

The total difference to me after tax is about £2,000 which is effectively an extra month or survival if I can't find work right away!

Thanks for your help.