If I ever resolved this in the past I'd forgotten how, so here it is for next time. Thanks to MrExcel for the cell format that makes sense of summed Hour/Minute time values in Excel. By default (at least in 2003 which I'm using at work) you get a meaningless number until the cell format is changed. Strange thing is the sum value in the status bar has the correct formatting/result!
I wonder why Excel 2003 doesn't apply this automatically?