Its best not to set a start line from the bank without an inner distance mark at a generous distance from the bank, no matter how well placed the start line would appear to be. The reason for this comes in the introduction to Part2 C of the RRS - At Marks and obstructions. This says "Section C rules do not apply at a starting mark surrounded by navigable water..." The significance of this is that if you have no inner distance mark then the part C rules apply, and boats are allowed to "barge" in at the start boat and ask for water.
The interactions of rules 18, 19 and 20 in this situation appear to me to get very complicated indeed (and R15 & 16 come into the mix as well), and I am not sufficiently confident to be able to offer a guide to who has what rights in the situation below. Instead I am just presenting a diagram showing the sort of positions that boats can get into, and what they might be saying to other boats.
The only advice I can offer for a situation like this is "don't let it happen". A lot of race teams seem to get hung up on setting the start line exactly on the final mark, and this can leave a start line which gets fearfully close to the bank and gives the less agile boats and less experienced sailors very little room to play with. I suggest that a start mark should never be set less than about 50 yards from the bank or another obstruction. If that means moving the start line 50 yards up the first beat from the last mark then do so. Even if you are average lapping boats then the tiny difference in distance cannot make any difference to the results, so its much better to set a line with plenty of space round it, which is safer and fairer for everyone.