OK well we have a definite maybe from MadridMan, I'm going to get tickets probably tomorrow for Monday, which starts at 11.00 am. Anyone fancy watching some tennis ?

The official web site is
http://www.madrid-open.com/site/start

It is outdoors, and the forecast for Monday is currently giving a 40% chance of some rain, but cloudy and sunny so not too bad.
http://www.aemet.es/es/eltiempo/prediccion/localidades?l=28001

Tuesday looks worse.

Just one note, I'm reliably informed that the top seeds will get a bye from the early rounds, and probably not start playing until Wednesday. So if you want to see Nadal, later in the week may be a safer bet. I may go Thursday too.