Hi Macman,
I agree with some of what has already been said, but also disagree with some of the advice.
As long as it's not blowing a hooley, fish from a drifting boat. As long as you can keep in touch with your flies you will almost always catch more fish, as you will be covering more fish.
10lb leader is fine. When buzzer fishing I use Sightfree Platinum 10lb straight through, whether on the bung or straight lining. Once you get confident with a leader material, don't be tempted to change - stick with it. It's very easy to blame your leader when it's usually something else that's the problem.
The most crucial factors (in order of importance) for catching fish on the big ressies are:
1. Area (If you're not over fish, you won't catch any)
2. Depth (If you're fishing under them, you've only got a chance on the drop or the lift/hang). Depth of water can also be important, generally aim to be over 10-15ft water.
3. Concentration (Keep your rod tip above the water surface and use the flyine coming from your tip ring as a swing tip to get an early indication of a take)
4. Fly (Anything buzzer shaped in the correct size will catch fish when they are hard on the buzzer, which they are at Grafham at the moment)
5. Retrieve (sometimes they want them static, sometimes sink and draw can do the business)
6. If other boats are catching well and you can't touch them, try approaching them very slowly from the side and politely asking how they're catching. Most people will be only too pleased to tell you, if only to show how clever they are!

)
Reading your post you didn't really seem to be doing a great deal wrong. I would suggest next time to start by using a bite indicator (dons tin hat) with your buzzers at 4ft, 8ft and 12ft from the indicator. Aim to keep a straightish line to the indicator without moving it unnecessarily. If this doesn't work then a draw of about 12 inches can sometimes induce a take.
Once you start to catch fish you should get an accurate indication of what depth they are feeding at. Don't be tempted to fish the indicator all day, you can now attempt to fish more than one fly at the correct depth by straight lining with a booby on the point. Knowing what depth the fish are at will give you a lot more confidence while doing this and you'll fish better for it.
You don't have to use anything too fancy fly-wise, in fact PM me your address and I'll send you some buzzers I have had a lot of success with on Rutland and Grafham.
Hope this helps,
Graham
I fished Grafham on Sunday in similar conditions from a drifting boat and had 20 nice fish from different areas.