Well tonight I finally managed to get the G0MRF transmitter running fine!!!
Great to get this far after all the frustrating weeks of experimenting
etc.........

I took apart another monitor HV-transformer and used this core.

I am not sure what caused the problem, but maybe it was because I used
an core with no air gap at all.
The new core has 2 very thin air gaps, because of the glue I used.
This gives lowe inductance, but anyway it works, and thats what counts.

Tests has been done at 20V drainvoltage only, and results are very
very promising, with efficiency over 90%!! (DCinput vs. RF output)
System behaves like an 6.2 Ohm resistor, which will give around 8A
drain current at 50V.
Next step will be trying to get the SWR protection circuit running, and
after that I'll go for 'power' tests on 50V.

I would like to thank everybody for their contributions, like answering my
questions and giving tips etc.

73! Dick, pa4vhf