Robin,
Removing both 'iformat' and 'Rowi' will solve the problem, but it's unexpected. Is this a bug?
No, I don't think this is a bug. The section of the Lout manual "Changing the appearance of rows" (Section 6.3 in my very old manual) explains that there are eight row format options. The important word here is *format*. If you want up to eight different formats for the rows in your table, you use @Rowa, @Rowb etc. If you want all the rows the same, they can all be @Rowa.
I've edited your table a little -- see below. I added alternating shaded rows with the apaint option. The final "|" after @Cell D alters how the width of the cell D is handled -- I can't remember why this is necessary. Someone else on the list can explain. You can take it back out and see what happens -- cell D becomes extra wide.
@SysInclude{doc} # Changed to SysInclude for test
@SysInclude{tbl}
@Document
@InitialFont { Helvetica Base 12p }
//
@Text
@Begin
@Tbl break{lines}
apaint{lightgrey}
address@hidden A | @Cell B | @Cell C | @Cell D|}
address@hidden A | @Cell B | @Cell C | @Cell D|}
{
@Rowa address@hidden@B{Due address@hidden@B{Paid}}
@Rowb A{1%}B{09 July 2013}C{58.80}D{0.00}
@Rowa A{1%}B{09 July 2013}C{58.80}D{0.00}
@Rowb A{5%}B{09 July 2013}C{29.40}D{0.00}
@Rowa A{2%}B{09 July 2013}C{11.76}D{0.00}
@Rowb A{8%}B{09 July 2013}C{47.04}D{0.00}
@Rowa A{2%}B{09 July 2013}C{11.76}D{0.00}
@Rowb A{3%}B{09 July 2013}C{17.64}D{0.00}
}
@End
@Text