As I said I was getting a 172 code: "System Weak"

The vacuum system appears tight (unless there is another takeoff I
haven't found) except for the feed to the Pulse Air and EGR system
solenoids. The pulse air holds vacuum, but the EGR doesn't. The EGR
therefore seems a bit hit and miss. Also, sadly, the manual I am using
says that it should hold vacuum. I've got a new one, which I am about
to fit. I will then see if I have a 172 code or not.

 Yes Neil, the EVR doesn't totally block vacuum when "at rest". It allows a bit of a
vacuum leak to atmosphere. When energized, it sends vacuum to the EGR valve. Even
then, it's not a "perfect" vacuum.
 I have to ask, why are you dismantling your EGR solenoid? Fault code, by chance?

Has anyone ever taken an Electronic Vacuum Regulator apart?

I was getting a 172 (EEC-IV) fault code, which indicates "system weak".
A vacuum pump revealed that the EVR solenoid (which controls vacuum to
the EGR valve) would not hold vacuum whether it was energised or not.
Clamping the vacuum feed to the EVR caused the 172 to go away (although
I did get a code inidicating "EGR valve not opening").

Taking the EVR apart I find that it has very few parts. A coil acts on
a metal disk to cover or uncover the outlet port. But, it doesn't seem
to be air tight. There is no seal around the disk. Also the disk is
held against the solenoid coil by a spring. I suppose vacuum might pull
it away from the coil but there is nothing for it to seal against. It's
seat even has raised lands to, it would appear, allow some leakage of
vacuum.

Anyone know what's going on?