twinkleymum, I don't know how much I can help - we seem to have similar problems! Gaviscon does give my DS constipation, and we try and 'underdose' to avoid this as much as possible. I know what you mean about the Gaviscon 'bounce' - I give some doses only to see them reappear seconds later, and clearly they haven't had time to do much.
I don't know how you dose - it's hard with a BF baby, because you can't mix in a certain amount 'per feed',like you can with a bottle. What I do is make up a sachet (approx 3 teaspoonfuls), and then use a small syringe to syringe feed DS about a teaspoon, or 5 mls, whenever I can during a feed. (DS takes breaks, so I try and get in there with a quick squirt when he's off the boob). This seems to work better than waiting until he's at the end of a feed, chockfull to brimming, then trying to get the full 3 teaspoons in. Which can often reappear very quickly.
The small syringe approach also means that if we get a small-ish vomit after a feed, I can give DS another syringeful straight away, and hopefully keep the rest of the feed down.
I seem to remember someone on the reflux thread giving some gaviscon before a feed.... I've never tried this, as I wouldn't want to dose DS, and then find he didn't want to feed after all, IYSWIM. But it's an idea.
Sorry not to be more help - it is a difficult and frustrating condition to manage, I really do sympathise.