The excessive content stream are objects like vector data which can't be downsampled. The best approach maybe to convert to jpeg and back to pdf, if you reaally need the files smaller.
Many thanks Dayle
In fact I was just following that line whilst you were answering.
PDF > Export pages as JPEG's (highest quality)
Import JPEG pages into InDesign (saved time by setting up ID doc exactly same size as original Powerpoint so JPEGS drop-in without scaling)
Output PDF on smallest setting
Result: 2.2Mb (from orginal 8Mb). Type not as sharp but acceptable price to pay for smaller file size.
I read your answer just after I completed this task.
Thanks once again ...great minds think alike!