Reason I ask a few weeks ago it was warm in my kitchen and the granache was far too soft to put the SP on - I did try it but it became a mess the SP and the granache came off and I reapplied dark chocolate.

For future use is White chocolate granche best avoided? I did 1:3 and the granache was fine when applied and chilled but after a while in my kitchen it just wasnt stable. If its warm will this white chocolate just no support the cake?

AI haven't had an issue with it. I use a 3:1 ratio choc to cream and let it set on the cake overnight before covering it with fondant. Haven't had a problem, even in warm weather. I do prefer dark choc ganache, but that's only from a flavour point of view - I only use white choc ganache in cakes where there is enough acidity eg lemon to balance th sweetness.

Maybe you didn't let it set up enough? Or perhaps you didn't use the right chocolate (I use Callebaut couverture) or cream (I use 35% fat)?

APlay with the ratios. There's a lady I know using a ratio of 4:1 for white chocolate ganache.

I use 4:1 (although I also try to avoid white choc ganache where I can as its so temperamental!).  Like Winniemog said, are you sure it had set up properly?  I was having huge problems when I first started playing with WCG because it just wouldn't behave anything like my dark chocolate ganache (ok in the fridge but yuck at room temp)...turns out it wasn't set up properly and had actually split a bit.  Got out the electric whick and beat it for a bit, added a blob of glucose and all fixed :-) Still...hate the stuff!