I don't understand why, but one of the K's Adams Radial splashers is too small. Quite a bit too small. Small enough to leave a great big gap around it's top edge when fitted in to the side that has a gap for it.
My suspicion is that the wrong part was in the kit and the previous builder just fitted it and bodged with filler.
That doesn't work as the riveted strip looks odd, it should be along a nice clean right angle.
Never mind. First I pushed low melt solder into the gap from inside the loco. You can use this stuff as both joiner and filler.
Then a bit of microstrip over the top of the splasher bulked it up enough. Files back to provide a flat face and all is now good.