Thanks for that.
I ticked the box in Webserver settings and that resulted in the TinyMCE being enabled in the web form. I could then enter text in the control, but when the form was saved, the text was not saved - reopening the form, the text control was empty. Unticking the XHTML box on the form makes it work fine in plain text. The text field is a Memo.
What am I doing wrong?