Compile error on customised NC Install - interface member not found

I am new to NC and hopefully this is something easily solved.

I have inherited some code that I need to modify. The previous developer assures me it compiles - I have verified code against codebase of working site and it matches.

I get a compile error on the Plugins\Nop.Plugin.Misc.WebServices\NopService.cs file.

Nop.Plugin.Misc.WebServices.NopService' does not implement interface member 'Nop.Plugin.Misc.WebServices.INopService.UpdateOrderShippingInfo ...

Obviously the compiler is saying the class does not implement a member from the INopService interface. However....

I checked the Plugins\Nop.Plugin.Misc.WebServices\NopService.cs and Plugins\Nop.Plugin.Misc.WebServices\INopService.cs files from the source download for the latest version of Nop - and they are the same as the code I have.

Issue appears to be that in class NopService UpdateOrderShippingInfo is implemented with different parameters from those in the Interface definition - but in the downloaded source this does not appear to be an issue - only in the code I have.

I did search the forum and have done quite a bit of googling but nothing.

I also compiled the Nop Sourced download and it compiles fine.

Any assistence would be appreciated.