The Dulux Satinwood paint is available in a solvent base and a water base. The water base is for walls and the solvent base is for trim and metal. I'm going to assume you used the water base Satinwood paint.
The emulsion paint should stick without the need of sanding or priming. I recommend you test a small area. Apply a little emulsion paint and allow it to dry for 24 hours. After drying, try to scrape it with a finger nail. If it comes easily you will need to prime before continuing. If none comes off or it is very difficult to scrape off, simply proceed with the painting.
An alternative to priming is a light sanding using 150 grit wet/dry sandpaper and a little water to keep the dust down. This preparation method will require the walls be washed after sanding and is a lot of work. Using a universal quick drying primer will do the same thing and is faster.