Hi Debster
I haven't been through this process, but I also work for a local authority and have responsibility for personnel issues for my section (I'm not a personnel officer though - just someone that personnel staff talk to and vice versa regarding personnel issues).
I take it that you are trying to move to a 0.8 post. Is there not an option for the 0.2 part of your FT post to be filled by someone else (ie it becomes a job share) so that the hours when you're not there, someone is actually covering your post. I'm not sure how practical that would be because you're talking about doing a 08.30 - 2.30pm day, and finding people for the end of the day can be difficult, but that is one suggestion you could make. If that happened, there would be no impact on the service you provide or on your colleagues or line manager.
As a part timer myself, I have to say that it is likely that your workload won't decrease significantly - I'm 0.5, but to do my job properly, I should really do at least 0.7, and that's about the level of workload I have. You need to be forewarned and forearmed about that one - by saying that people will have to wait until you get back, you are effectively agreeing to fit your full time workload into a part time job - be careful!!
Another thing you could mention under 1 is that by moving to part time, you will be less stressed than if you have to remain at full time, and the impact of that will be a better service for your department....it's a positive!
Under 2, it depends which department you work in. For example, if you work in education and have to liaise with schools, the fact that you're not going to be there at the end of the day won't be a big deal. Can you demonstrate that there is a bigger workload in the first 2/3 of the day? - ie do you get many more phone calls during that time, do you get many more e-mails during that time, are meetings usually in the morning?
Another thing you could say under 2 is that you would ensure that you send all the people you work with (external and internal) an email explaining your change in hours and when you can be contacted, and you would change your e-mail footer and voice mail to say when you're available. By doing that you will be minimising the impact of you not being there in the afternoon because everyone will know and theoretically won't expect a response immediately.
If you're in a managerial post, you could suggest that some more admin support (perhaps paid for in part out of the 0.2 salary saving they are making from you) could help....or maybe that's a bit cheeky!! In my department, that would be laughed out, but I'd put it in anyway to make a point! (under staffed, under resourced, massive budget overspend....you know how it is!)
Remember that you have a legal right to make this request and they have to take it seriously. If you're really worried about it, speak to someone in your personnel department for guidance - they should help you if you have specific queries or concerns.
Hope this has helped a bit!