Gradle: How to set version from custom plugin - gradle

Question

I've written a custom plugin (my first) which manages a "build number" which I want to include in the build.gradle "version" as so:
allprojects {
version = "1.2.3.${buildNumber}"
}
Unfortunately my plugin is never run and thus the buildNumber returned is "null".
Below is my build.gradle.
If I run "gradle showbuild" I see the right buildNumber.
If I run "gradle showInfo" the build number reported is "null".
Somehow I need to get gradle to call my plugin task 'buildInfoLoad' before the "version" value is set in allprojects. Since I'm knew to gradle I am struggling with this.
Any pointers would be most appreciated!
buildscript {
repositories {
maven {
// Access to MangoGrove
url uri('../repo')
}
maven { // aka "jcenter()"
url "https://jcenter.bintray.com"
}
}
dependencies {
// Provide the "provided" and "optional" methods for dependencies
classpath 'com.netflix.nebula:gradle-extra-configurations-plugin:3.0.3'
classpath group: 'org.mangogrove.gradle',
name: 'MangoGrove',
version: '1.0.0-SNAPSHOT'
}
}
apply plugin: 'org.mangogrove.gradle'
task showbuild(dependsOn: 'buildInfoLoad') << {
println("BI build number: $buildinfo.buildNumber")
println("BI build time: $buildinfo.buildTime")
}
task createbuild(dependsOn: 'buildInfoCreate') << {
println("BI build number: $buildinfo.buildNumber")
println("BI build time: $buildinfo.buildTime")
}
allprojects {
ext {
buildNumber = "$buildinfo.buildNumber"
}
version = "1.0.0.${buildNumber}-alpha2"
//version = "1.0.0.${buildinfo.buildNumber}-alpha2"
apply plugin: 'eclipse'
eclipse {
classpath {
downloadSources=true
//downloadJavadoc=true
}
}
}
task showInfo << {
println("Product Version is $version")
println("Product BuildNumber is $buildNumber")
}