Peter, sorry for the delay replying, been a bit distracted.
Yes I am aware of the dual servo but I feel the car being so light, circa 700Kg and my being well used to non servo brakes, even prefer them, it shouldn't be an issue.
Thanks for the part number of the seals, I will follow that up, if my local dealer can't (or won't) supply is there an on-line source of BMW parts?
For now I am going to try the conventional layout of the rear piston for the front brakes and the front piston for the rear brakes. I think the rear piston area will be reduced by the shaft which links the front and rear pistons, so that will reduce the effective piston area. I may even do some pressure tests just to see what happens.
Now for the bad news, I have found that my 'best' engine block isn't so good after all. I have been de-greaseing and cleaning it carefully. Now it's cleaner, I have discovered the bores are badly scored in their lower half's and worse, #8 cylinder sleeve has dropped by 12 thou. My engine man isn't too concerned about the scoring but suggesting topping the block on both sides to get the deck down to the top of the liner. We discussed trying to pull the liner back up the block but I am apprehensive about that it seems it can take 20 tons force to move them, and that's down, with a good face to push on. To pull one up would need a special dolly and there is very little room to play inside the crankcase.
I do have another SD1 block, so tomorrow I intend to clean it carefully and see which is the better block.
Otherwise making good progress, crept over the 60% complete this week, I think Easter is a realistic target.
