In your situation a better part of the DirectGov website (that Dot refers to) is here:
http://www.direct.gov.uk/en/Employment/Understandingyourworkstatus/Temporarylayoff/DG_10026693
I suggest you read particularly the bit about the difference between being 'laid-off' and being put on short time working. In most cases, an employer cannot lay off an employee without continuing to pay salary normally. The main exceptions to this are if it specifically says so in some documentation from the employer, including the Company Handbook (if there is one) or if it generally applies in your industry. Neither are likely to be applicable to you. So your employer seems to be asking for you to agree a change your contract from 3 to 2 days - either permanently or temporarily. You do not have to agree to this. You could point out that there are 1.6 people-days of this work being done at present so it would be far fairer to reduce the full-timers from 5 to 4, rather than yours. If indeed the jobs are broadly the same. Or you could suggest 0.5 days each reduction.
If this is a permanent proposed change in your contract, your employer needs to give you notice - 8 weeks notice in your case since you've been there over 8 years. If it is temporary, the notice period does not apply. I would agree that it seems rather unfair that you are being singled out for all the pain.
The answer to the second part of your question is 4 weeks - if you are temporarily put on short-time working for four consecutive weeks, you can claim redundancy has taken place. See the website link for more information - YOU have to assert the redundancy.
A later redundancy would be based upon the 24 hours per week contract - any agreement with you to reduce your hours to 16 per week is only a temporary measure.