Gas hot water system

Hi me again just had our first night without being plugged in and tried to get the hot water system working and followed the instructions in the manual but it just kept failing to fire up and the green light turned red. Any ideas what I am doing wrong

Did you remove the flue cover outside?

No it said something about windows near flue ?

I'm assuming a 2003 Duetto will have a Truma water heater which has this type of flue cover which needs removing

Look at this on eBay  http://www.ebay.co.uk/itm/263052596849

I'm unfamiliar with the details of the Duetto, but it is a requirement that all conversions should be fitted with a cut-off switch on any opening window fitted above the flue exit from a heater.  If you have one on the Duetto it will prevent the heater from starting if the switch is "open".
They are usually (not always !) a magnet proximity operation on a "normally closed" switch
This, of course, is only one of many possible reasons for no-go.   Keith