Best price ever the camel tells me, at this price I'd buy two (or something else to meet the £20 free delivery threshold) if you don't have Prime.
Awesome "mixing" rum at the entry-level end of the spectrum from Mount Gay (personally I'll still drink this as sipping rum as it is a cut above, certainly can't beat it at this price).
I believe they're eventually looking to phase the Eclipse out (in favour of the Black Barrel) so probably best to buy 12 bottles just to be on the safe side.
Also, worth noting that Ocado are doing this for a mighty 10 pence less if you are able to use them.
Drink up, me 'earties, yo ho!