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Unfortunately all the editors were already set to dhtmltextarea. I tried all the other options to no avail.
The module throwing the error is a "content" module (of which there seem to be a number) by a Ben Brown of The Handcoders, Reinarz & Associates. These seem to have disappeared now.
I built the site for a charity in 2010 and it has just run lovely since then, but it was on an old PHP version which has had to be changed, so somehow I need to get it changed at least to the point of being able to edit the content. So yes is it old.