Problem with IDispatch and ActiveX controls

I'm having problems trying to use a third party ActiveX control in C++
Builder 6

The ActiveX control was developed using Trolltech's ActiveQt library. Such
controls are dispatch-only, ie. no dual interfaces, so everything goes
through IDispatch::Invoke().
The Trolltech ActiveX controls do work in Microsoft containers such as VB6,
Internet Explorer and MFC.

The controls work OK at design time - they are imported into a bpk which
compiles OK. They can be placed on a form and sized.
At runtime the controls are instantiated OK and properties can be set, but
calling a method causes an assert "m_dispatch == 0"
followed by a crash.

Trolltech believe the problem is in Builder's support for ActiveX.
Anyone here encountered similar problems?

Thanks,
--
Paul