Answer

I bid 6 Spades. East has shown long diamonds, probably 6. I have 4 diamonds. West has supported diamonds, probably 3. So Brent probably has a void. He's also bid spades at the 4 level on his own and I have Qxx of those with two outside aces.

5 Spades would surely be a mistake. There's a small chance he doesn't have a void and there's also a small chance he has a bad holding in hearts (although he's got to have something in the side suits otherwise he can't be strong enough to open 2C since I know he has no points in diamonds).

So I'm feeling ok about things and it's passed to East who bids 7D! Now what?

We're Red vs Green so although we can take good penalties against 7D it's almost certainly not going to be as good as 7 spades if that is making. I'm now even more sure that Brent has a diamond void.

After a long think I bid 7S and cross my fingers.

Right hand opponent leads the Ace of Diamonds and my comment as my hand goes down: "If that's cashing, we're off!". Huge relief when Brent replies "It's not" and then goes on to make all 13 for a big score of 2210 (One table was in 7Dx going off 5 for 1100)

The moral of the story: Beware when supporting partner in high level pre-emptive / sacrifice hands. You might just give the opposition more information that will get them to a better score. We would probably have found the small slam anyway but when West supported and then East bid again it made life much easier (albeit a little scary) for me to bid the grand.