Not one of my better purchases: too tart. Avoid.
I’m not the first to say this, but if you’re looking for something white and fresh, look to New Zealand and South Africa for Sauvignon Blancs.
Also, I just had a white from Spain that was quite floral and a little spicy that might be what you’re trying to get from that Sancerre. I don’t have the bottle with me, but it was called “Rueda Verdejo”. I think 2004. I made a note to get it in the summer. I opened it because I was making buerre blanc the other night and it was one of the few whites I had sitting around.
A good wine I’ve had recently: Chateau Carsin 2000.
Apellation 1er Cote de Bordeaux Controllee.
My wine store had it on sale. I bought one bottle to drink, then went back and bought 6 more, and I’m planning on drinking one per year for the next 6 years.
Sancerre is one of my favourite wines. I generally prefer French wines anyway, which is a good thing as I’ve still got 170+ bottles of French wine to go from my last visit to Calais.
Is there a chance it may mellow with age?
That bottle won’t. 
I’ve got another, which I’ll likely try in a year or so.
My absolute favourite white wine has to be Pouilly Fumé. There are better wines - the Montrachets, for instance - but they’re rather more expensive.
How do you feel about German whites? I know some people find them too sweet but I kind of like them.
I have very little experience. The nearest I’ve got - apart from the rock-bottom quality stuff like Hock - is wine from Alsace. I found the Pinot Gris not to my taste, but the Pinot Blanc is very yummy indeed, and the Pinot Noir is almost a rosé and goes wonderfully with lamb.