Hi Michael
We don't use conditions but we do issue split decisions by using two sepearate decision notices. The first permits that part of the application which is acceptable, the second refusing the offending part. Inspector's don't have anything adverse to say about this - when it comes to appeals they set aside the part of the proposal which was permitted and focus on the refused part. Obviously this only works on easily separable developments - for example we may permit a rear conservatory but refuse the front porch under the same permission.
We put a note on the each decisio notice to say that this forms only one part of the LPAs decision and that the part of the proposal relating to X was permitted/refused.
Hope that makes sense.
Not aware of any guidance but if it's good enough for the SoS it's good enough for us! I guess PINS may have some guidance?