For the seven years I have been working here (for the same Thai Company) on renewing Visa and WP, there has usually been issued a 1 month Visa whilst the application is under consideration.

In the past that has not been a problem, but this time I will have to do an overseas business trip whilst 'under consideration". In the past I have always gone for a multiple re Entry but in this case would it be prudent just to apply fo the single entry OR will a multiple be carried foreard onto my new Visa if approved ?

It would not be carried over. You need a single entry for your under consideration stamp and when you get the full extension of stay you can obtain the multi entry (if you need it).