I have been given (lucky me) a magnum of 1996 Chateau Margaux Pavillion Rouge and would like to know whether I should drink it now or will it continue to improve with age.

If the latter, then when is it best to drink?

Pavilion Rouge de Chateau Margaux is Chateau Margaux (http://www.chateau-margaux.com/)'s secondary label.
This is a truly awesome wine.
1996 was a good vintage and will keep well.
Now normally I'd say keep it for 15-20 years to get the most from it, but as it's a mgnum, I'd say look closer to 20+, so drinking at it's peak would be from 2016 onwards.
Wines in magnum (1.5 litre equivelant to 2 normal bottles) mature slower due the there being less air in relationship to the volume.
Alot of classical producers say that magnum bottled wines are the best for long term storage.

Now as to storage you'll need to find somewhere dark, vibration free and constant temperature around 10 - 12 degrees C. Under the stairs is NOT good for long term storage and certainly not the kitchen as the temperature varies too much over the course of a day.
Best stored on its side to ensure the cork doesn't dry out and then the wine will oxidise quickly.
Did it come in a box, if so you can safely store it next to the house wall of a garage, off the floor.