[Amps] 3 phase transformers on single phase - advice pls?

[DAVE WHITE]

Guys
 
I'm sure I should know the answer to this, but just can't seem to find anything in my literature or on the web.  A friend (G7ODR, cc'd on this email) and I have scratched our heads, but the more we discuss the problem, the more complex it becomes.....
 
If I use a 3 phase transformer (either a 380v or 415v primary) on a 240v single phase power supply,  then what will happen in the secondaries?
 
I think the transformers that I have access to are delta input and star output.  They're open 3-bobbin jobs rather than potted or enclosed.
 
Can I wind the secondaries in series without overloading the insulation?
Could I wind them in parallel?  Would I hit core saturation problems?
 
So, to put a few figures on it, if it's a 12kva 3 phase 380v in, 6kv out, what would happen with the various permutations if I try to wire it onto 240v single phase?
 
I'm actually looking for 4.5kv RMS out at 10kva and can get a single phase one custom wound, but at a price (£950 from TEC).  Anyone in the UK know of a cheaper but quality source?
 
Phone a friend, ask the audience etc.... ;-)
 
Any ideas appreciated.
 
cheers
 
Dave G0OIL / K2MOO