I need to replace the water pump and timing belt on a 63 plate 2.0 TDI 150 BHP engine. When looking at the various kits available, I have noticed that some kits sell a non switchable water pump and some sell a switchable water pump. How do I determine what water pump I require? 

OE pump is switchable and is sold as complete unit (pump with electromagnetic valve).

 

Aftermarket pumps can vary depending on brand:

 pumps without valve; then you use old valve;

 pumps with valve;

 not-switchable pumps (not OE solution) for those who want avoid possible sticking of impeller's sliding "shutter ring" with following overheat issues.

Just my personal opinion, I would use this version of pump only in hot country.

Otherwise warm-up time of engine and emission quality (for those who cares about it) will be influenced.