Thank you all for your advice. I've just ordered the JC Whitney carby synchronizer and passed on the posts to Dad. He's located the idle air bleed hole on the carby but hasn't yet checked the balance and idle strength.
I re-visited the Vintage Mercedes forum and gmasks' thread "Diagnose Dual Zenith Carbs 72 250 engine" which identified Baum Tools (www.baumtools.com
) as the supplier of the carburetor hood but also provided details about adapting a Rubbermaid food storage container to do the same job.