1. I would not have filled the radiator completely. The working
pressure of the radiator is 10 Bar. I do not believe that a 400w heater
immersed in oil with an air allowance for expansion would manage to
create that pressure. I would also have left the bleed slightly open
for a while to confirm this. I was also considering thermostat control
to prevent excessive temperature, but the radiator should have been
able to dissipate enough power to obviate the need for it anyway.

2. Cost not an issue, warmth is!

All a hypothetical argument, as the 90 degree turn into the radiator
scuppers the idea. Are all radiators made like this?

I'm going to use this instead, as it has a thermostat and is designed
for the job.
http://www.screwfix.com/app/sfd/cat/...24433&id=69854

Thanks all for your help and advice