Sugar soap is good but wouldn't use it unless you are going to repaint them afterwards as it may take the shine off them. If you are not repainting then just use washing up liquid as zzxxee says or a general purpose detergent but not a scourer of any type as this will take the shine off.
Any liquid soap should be fine. Rinse it afterwards with clean water to which a drop of vinegar has been added which should remove any traces of grease.
Many Thanks All...I should have made it clearer that the paintwork was to be repainted, just to clean, so I think liquid soap and water with vinegar or detergent will do.