I agree with Smirnoff. I took this cheap FRA-LHR-SIN in F earlier this year and if I'd been stuck at FRA, I wouldn't have dreamt of demanding BA to rebook me on a much more expensive LH or SQ F ex-FRA.

Similarly, when I got shafted by FR a number of years ago, I didn't expect compensation given that we'd paid £40 for the return flights to Italy for the two of us in total!

You get what you pay for (and in the OP's case that meant taking the next available flight the following evening)!