don't re-invent the wheel
the genuine lister start-o-matics addressed all these problems
http://www.surfbaud.co.uk/Lister/
We're not reinventing the wheel. Our "road" is a little bit different, so we need a different "wheel" than you do. Our voltage here is 120/240, and we have special requirements for bonding and grounding. Genuine Listers wouldn't be able to address either issue, and eletrical inspectors here would never approve such a unit without some serious overhauls. Although UL & CSA, and US & Canadian elctrical standards in general are accepted almost all over the world, the reverse certainly isn't true. Inspectors over here will just laugh at more European electrical ratings, standards and codes.
I like troys idea, even though I had to read through it a couple times. I drew this little diagram of what troy was explaining:
DETROIT {~}----> [120/240 PANEL] ---------->>---------- [X-FER SWITCH]-->>----->[120V PANEL]
| | |
| ^ |
| | |
| {~} |
240V and BIG LOADS LISTER SMALL 120 V LOADS
It is simple, and offers the least posbility for screw-ups. We COULD put a transfer switch before the 120/240V panel, but it will need to have a higher rating, and then there are some "ifs and buts" about hooking up only 120V from the Lister. I the above scenario, anything you want to run off the lister would have to be on the 120V panel (including the battery charger for your detroit) and your bigger loads would go on the 120/240V panel. My {~} indicates a fuse or breaker right at the generators, and I'm assuming that your 120/240V panel will have a main breaker and a breaker feeding the X-fer switch. The small 120V panel wouldn't require it's own main breaker in this scenario, but I'd use a breaker on the lister as it's easier to reset.
If you're interested in this setup, we can talk about grounding and bonding in more detail. Whatever you do, make sure that you only have one ground. (Ie. dont put in a ground rod in one spot, and another one 50' away and ground half the building with one, and the other half with another, or even tie them together. I'll say it again: ONLY ONE GROUND) Of course, you can put multiple plates/rods down in the same area, and infact that is recommended. But they should all be attached with the same wire.
Steve