Thanks guys.
The masking process is pretty straighforward for the basic type of crosses. I lay two pieces of Tamiya's Kamoi tape onto a cutting map (so I can get two crosses in one go) and then put the decal I want to copy into warm water as you would normally. The decal is then applied to the tape and blotted to remove excess water. A third piece of tape is then laid directly over the top.
The decal shows clearly through the tape and I then cut the cross out with a brand new scalpel using freehand for the curved sections and a steel rule for the straight parts. I then use a rule again to cut out the outside of the square.
The white square is masked on the kit and painted and left to dry. The top layer of the mask is removed and discarded (will have the now unwanted decal stuck to it) and carefully remove the next layer from the cutting mat (leaving one behind for the opposite side) and apply to the fuselage using the outer masking tape as a template for correct alignment. I then add thin strips around the edge to cover any small gaps there may be between the mask and the outer tapes....

Black is then airbrushed on. Important to use thin layers here so it won't leave raised lines at the edges but don't over dilute the paint so it won't creep under the mask....

The mask is then carefully removed....

... along with the rest of the tape and you should have a nice Maltese cross on a white square....

Yes, it is a lot more involved than using decals, but you really do get a much better finish IMHO and no need to worry abut the decal not setting properly over raised details etc.
Gary