Simon,
In re-reading your original question, I came to the realization that in referring to the "formula window", you might have meant the "Formula Bar" (Excel's term) located at the top of the worksheet. This is where a formula shows when you place the cursor on a cell which contains data (either a formula or text).
If indeed the Formula Bar is what you were referring to, then the solution is as follows: On each row, at the point where you in effect want to "force a return", simply use "Alt - Enter" - i.e. hold down the "Alt" key and hit the "Enter" key. This will force the cursor to the next row, and permit you to place segments of your formula on different lines - to make a long formula easier to read, as you pointed out. Good idea.
I hope this is what you were looking for. Please advise.
Regards, ...Dale Watson dwatson@bsi.gov.mb.ca