Ok,
I started in the AppleScript Editor and New from Template for the Mail options.
I removed the comments (to make bits active) and removed bit I didn't need.
I then added the tell Messages app bit.
Whilst it acts like it runs I am not sure why it does not. (? the Messages section in th wrong place ?)
using terms from application "Mail"
on perform mail action with messages these_messages for rule this_rule
tell application "Mail"
set the message_count to the count of these_messages
repeat with i from 1 to the message_count
set this_message to item i of these_messages
set this_sender to the sender of this_message
get this_sender
try
set this_subject to (subject of this_message) as Unicode text
if this_subject is "" then error
on error
set this_subject to "NO SUBJECT"
end try
try
set this_content to (every character of content of this_message) as Unicode text
if this_content is in {"", "?"} then error
on error error_message
set this_content to "NO CONTENT"
end try
end repeat
end tell
tell application "Messages"
set myid to get id of first service
set theBuddy to buddy "+44iPhone Number" of service id myid
send "You have received Mail from " & this_sender & " regarding " & this_subject & " which says " & this_content to theBuddy
end tell
end perform mail action with messages
end using terms from
The First Service is what is top in the Messages > Preferences > Accounts list
On most peoples it will be the iMessages Account.
It is in Mail as a Rule which is set to a date limited check and a specific email sender.

I still feel I am missing something.

10:45 pm Friday; July 4, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad