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!