Sorry Robin, too busy swapping 432 packs!
As has already been said, this is more about current than voltage, although voltage can help understand what's happening.
Firstly, what sort of batteries does the Supacat have? All MV batteries are not created equal even though they may look the same. For example, the conventional 6TN flooded cell battery will only deliver around half the starting current (CCA) of a 6TNMF Hawker. It may just be that the Supacat batteries can't deliver the sort of current required - a stalled starter on a big diesel needs a LOT of current!
Secondly, what state are the Supacat batteries in? I'd expect them to be around 26V to 27V fully charged, rising to 28.5V when the engine is running. If it's any lower than this I'd put them on a battery tester to check. Failing that, the alternator may not be putting out the correct voltage to keep them fully charged.
There may also be a problem with resistance between the Supacat's batteries and the starting socket, either in the earth line or possibly even the positive one. The only real way to check for this is to use the starter lead whilst simultaneously monitoring the voltage at the Supacat's batteries and the Stormer's. Any appreciable difference between the two suggests a poor connection somewhere.
Andy