I don't think it's a daft question at all. In fact, the strategy is a smart one. You may need to use a local BKK travel agent though, rather than doing it online.

As an example, I can buy an EK SIN-MEL-SIN for A$623 (via the EK SIN website). If I then log out of the SIN website, and check a MEL-SIN-MEL (via the AU website) it prices it at A$861 (which is nearly a 40% differential)

Just make sure the BA website gives you the best deal.