I was going to use a BMC4266 from Brakes International.
http://www.brakeparts.co.uk
Its a Girling cylinder that seems to be the most common one fitted to the Sierra range. It also seems to also be the one that is favoured by the Locust builders and various other "Seven" lookalikes.
It has three outlets - two at the furthest away from the servo for the fronts and the one closest to the servo for the rears. At 22mm diameter it is fairly close to the BMW 20.6mm diameter. Outlets are 10mm x 1mm which I think is the same as the BMW. Two face towards the engine and one downwards.
Biggest pain for me is that I have only one line going to the front at the moment and so will have to do rather more re-plumbing than I anticipated.
Robin