Thanks Mike and this is why I took a long time to think carefully about this solution. There are many ways to tackle these things all with varying degrees of success. I even considered having some kind of tri-layer polycarbonate moulding made but thought this would look better (it does!).
The temporary solution used by myself and the previous owner was an inflatable dingy which was sitting covering the old single glazed hatch. That actually worked perfectly and was also a good insulator but was a bit unsightly to say the least!!
As prices have already been mentioned, for the sake of completeness I should say that should it be neccessary to change the internal window - a vacuum double glazed unit of these dimensions (1.6m x 1m) is around £200 but to be honest I don't think it will be required.