The original version of 'Dancing In The Moonlight' was released by American band King Harvest - their version did well elsewhere, but was not released over here.
Toploader released it as their first single, and it didn't make a massive hit, but was re-released after their career took off, and was a hit the second time around.
A rather similar-to-the-original re-mix of the song has been released by a Norweigan band called Stargate, so if the original, the first version, the second version, or the re-mix of this particular song strike a chord in you, it could explain why you are feeling a sense of familiarity with it.