OK, I read up about it and it says (approximately in several places):
The bleach and ammonia contained in washing-up liquid break down the wax that is applied to the paintwork to protect it.
Once that protective layer is damaged, modern water-based paints can turn dull and, in extreme cases, start to crack.
Wax? Not on my bike. So not a problem.
But my favourite bit of info out there is this one:
Does washing-up liquid damage your vehicle?
We asked Dr Bob Eden BSc MSc PhD MICorr (Member of the Institute of Corrosion) for the truth:
“Washing up liquid does contain a ‘salt’ but this is the active ingredient and should not be confused with road salt. There is nothing in a washing-up liquid that will exacerbate corrosion – there’s no sodium chloride salt to worry about. The issue regarding corrosion is the ‘chloride’ bit of the salt. In ‘chloride nests’ at the base of a corrosion pit, the chloride exists as hydrogen chloride, which in damp conditions creates a solution of dilute hydrochloric acid, and it’s this acid that does the damage. You need to avoid ‘chloride’ from any and all sources, e.g. seawater, road grit and fish & chips (but not washing up liquid). When I wash my aluminium bodied Lea Francis, a dash of Fairy is just fine…â€