In the first instance, look here - there is some really, really good advice in there.
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029491
Feel free to ask another question if you are still unsure, but here is my take on it.
Firstly, painful though it is, I would get your probationary period over with. It shows your employer a level of commitment. And at the end of 6 months it opens the way to any rights you may have to ask for flexible working under the current statutory legislation. Please understand that the statutory legislation gives you the right to ask and puts the onus on the employer to say yes unless there is a good BUSINESS reason for not granting it - but as you will see from the list, there are only a limited number of personal situations where one can ask under the statutory legislation. This list may be apply to you.
Secondly, in 4 months time, I would explain the situation to your employer in a positive way so the employer can see the benefits. This is true whether you are applying under the statutory legislation or not. It may involve suggesting that you would like Xday off each week off normally, but that this could be varied to be another day occasionally if another member of staff was on leave, say - in other words you are prepared to be flexible about the arrangement.
Thirdly, unless you really want 30 hour day and not a 37.5, you could initially offer to work the same number of hours but over 4 days.
Much of what you may be able to agree depends on your employers approach to this (colleges and public sector organisations have embraced this more readily than some short-sighted private sector organisations) and depends also on what you do and what 'cover' you provide for incoming requests (for example, if you answer the phone to customers as