Solved!!
Someone who has been in the spa before me as hung the drainage pipes higher than the outlet of the ice bucket, as the pipe is connected to the rear of a jet the pressure was greater from the jet, against the weight of the water in the bucket, hence it fills, lower pipe, fit oneway flow valve, problem sorted.
I had the same issue with the drainage pipe from the filter housing, which also wasn't draining, used some soda suds, an air line to clear muck out, lower drainage pipe, all good.
cheers Rob.