Did you know you can also use a Morris Minor brake master cylinder? Whats the difference ? Apart from £60 its the bore size, 7/8 verses 13/16, yes a whole 16th of an inch. This might get you out of trouble till you can get the correct unit.
The upper master cylinder in the picture is a Morris Minor type (note the missing spring plate washer) its down the back of the work bench somewhere. On the left hand end of the large spring is the one way rubber cup valve which needs to be removed if this master cylinder is used as a clutch master cylinder.
Lower unit is the original JB type showing the layout of parts.