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> > Keith Smillie fancied a Differential Analyzer build from Meccano
> > parts, but found it easier just to model it.
....
Here's a Meccano differential analyzer.
http://meccano.us/differential_analyzers/robinson_da/index.html

Tim Robinson, who built it, also built a couple of Babbage difference
engines.  e.g.
http://www.meccano.us/difference_engines/rde_2/index.html
Tim is in thick of preparations at the Computer History Museum in
Mountain View, California, USA to show a 5-ton Difference Engine no. 2
built by the Science Museum of London:
http://www.computerhistory.org/babbage/

http://www.computerhistory.org/virtualvisiblestorage/popup_image.php?base_name=102637025
It's more impressive than one might expect from the picture.

If you're around London don't despair if you want to see it.  The
Babbage engine at the Computer History Museum is the second one built
by the Science Museum and is brand new.
Curtis

P.S.
Tim used Mathematica, with its extended-precision arithmetic, to
generate the starting values to put into the engine to demonstrate use
of the difference engine to make a log table which can be compared to
one published by Babbage himself.