Ted it is dead easy mate. All you have to do is make sure that the rubber bung and the plastic washer are put onto the walbro or else it will cause your engine to det on high boost.
The O ring on the pump housing is a little fiddly but if you keep rotating it clock wise whilst rotating the pump housing anti-clockwise then it will work it's way out from the tight casing that it is fitted in.
If I had pics then it would be easy but sods law and all that............
El