[Cruisin Marine]

ps I wonder if it would need to be a non polarised cap, it should not but I could see a lot of stress on a standard electrolytic.

I just see it as a coupling capacitor and if the DC conditions are always more positive at the feed end than the load end then a non-polarised would not be needed, unless I am missing something?
Knowing the AC handling capability of the cap would help.
Having said that, a non polarised cap would put my mind at rest much more.
I agree 90% mod. would be a nice figure to achieve.
The other factor to note is that you may have to increase the HT to achieve the power output after introducing that R-C cct. That all depends on whether you want to keep it at it what you originally intended of course. It all is down to how much headroom you have to play with really.