I got my new intake manifold from http://www.autolinkmx5.com/ yesterday.

It's from a 2001 MX5 1.8 Sport and has the same manifold as the JDM 1.8 NB RS.

I took some pictures and did som measurements.
Here is the data:

Weight: 6.4 kg
Throttle body opening diameter: 59mm
Runner diameter: 50 mm
Runner length: ~ 30 cm
Plenum volume: ~ 1.5 l
Calculated runner volume: ( 3.14 * sqr(2.5) * 30 ) / 1000 = ~ 0.59 l each

Here are some pictures and my observations:
All pictures can be watched in higher resolution here:

Testing Plenum size by using a plastic bag and filling it with water:

Exterior pics:

On the inside, both halves:
The 4 runners have a diameter of 50 mm.
The smaller of the 2 holes to the left on the upper picture under the runners is for EGR and has an internal channel that brings the exhaust from the right side to a small pasage on the top half of the manifold that exits just after the TB. The EGR was blocked on this manifold with an OEM blocking plate. Kind of strange that a UK car don't use EGR. I'm happy with that as my ETD manifold don't have EGR anyways.

The other slightly larger hole is the inlet to the Helmholtz chamber which is probably tuned for some RPM range optimal for the BP. It has something to do with reversion and canceling of pressure waves which optimizes flow.
Both halves makes a passage that starts on the right side and travels across to the left side where it goes into the Helmholtz volume between the plenum and runner 1-3. There seem to be nothing special inside the Helmholtz chamber but for some reason there are a small port of approx 2mm going from the chamber to the runners just before it enters the head. I guess it's some kind of fine tuning pressure or waves... Would be interesting to install NOS nozzels there instead..

The plenum looks nice and tidy with rounded inlets to the runners:

The neck from the Tb to the plenum is much straighter and less restrictive than my VTCT manifold. The opening is 59 mm but the neck is closer to 63-65 mm. Im pretty sure I can port the inlet to the same size which would be perfect for a Honda B-series throttle body which is somewhere around 62 mm compared to the 55mm (or is it 52mm ?) Mazda TB.

If I have enough time tomorrow I'll do the same measurements on the VTCS manifold for comparison.