Chris
I did this - increased the outlet to a 22mm - to suit a pressure washer add on (as and when I need to use it!).
If I remember rightly they come with a 15mm elbow - with a 22mm screw. I just replaced it with a 22mm screw in elbow and using a t junction I have managed to fit my pump with a 15mm reducer and have the 22mm pressure washer supply on the same line.
When I'm not using the pressure washer it has a tap connector so I can take it off without affecting the flow.
Does this make sense!!
Rob