I want to place planning site drawings (not geocoded) onto an Ordnance Survey basemap?

Have you looked at the RasterGeoreferencer?

For similar situations I use the AffineWarper.

 

The ungeoreferenced image goes into the Observed port.

The control is a set of 2 vertex lines that have the first vertex as the reference point in the image in pixel space, and the second vertex as the same reference point in projection space.

 

So a pixel at R17,C249 might correspond to 34.175N, -127.643W and a pixel at R1024, C30 might correspond to 31.145N, -145.261W.

 

Your control would be two lines

 

[(249,17),(-127.643, 34.175)]

[(30,1024),(-145.261,31.145)]

 

 

This assumes that your site drawings are more or less internally consistent. If the drawings are not to scale, then the technique you are looking for is called Rubbersheeting and FME does not currently support it for rasters.

How do you set up the control. Tried as csv but not sure how. This raster working is new to me.

 

 

 

You would either use a Creator transformer to hardcode the values or - if you could get the info from somewhere else (like a CSV file) then use a VertexCreator transformer

 

 

How do you set up the control. Tried as csv but not sure how. This raster working is new to me.

 

 

 

I have my my reference points in a CSV, I read it in, then use a vertexCreator (replace with point) with the image space coordinates, followed by another VertexCreator (Add point) with the projected space coordinates.

 

 

This gives me a line for every reference.