I made a leap of faith and ordered the Dual 7" servo from Ducksville yesterday, and it arrived at 9am this morning.
The above shows that the trial fit is very good news. The studs attaching to the pedal box are on 85mm centres compared to the Metro ones at 90mm, so only minor hole fettling is required to make them fit.
The maximum diameter around the seam is identical to the Metro servo, so it fits the space available between engine and side panel.
The length front to back is 2" longer than the Metro, but the secondary chamber is 1" smaller in dia., so remains inside the bonnet line.
I am reasonably confident that after I have made a new adaptor plate and extension pushrod between servo and master cylinder it will still fit under the height of the bonnet.
And if I can get away without new brake pipes it will all have been so easy its untrue.
I am well chuffed - Thanks Jason for a very good spot!