"If they do fall short of a majority who will they form a coalition government with? Labour? LibDems? BNP?"

BNP don't come into it as they have no MPs. Lib Dems would normally slide more towards Labour and the Tories have said they wouldn't countenance electoral reform so that looks unlikely. Outside of the various Ulster Unionists I can't really see who else would work with the Tories and it is unlikely that they would have a majority even with DUP support! Even though they are Labour's main rivals in Scotland I can't envisage the Scottish nationalists backing the Tories. Scoland has so turned its back on the Tories that Salmond surely couldn't countenance backing them and he has said they would vote on issues rather than go into any coalition. So I can't see the Tories getting a majority though it looks unlikely that any Lab-Lib coalition would have a majority either. Most likely eventual scenario if the Tories can't get the Lib_Dems to back them is perhaps Cameron running a minority govt!