Dear all,
Thanks for your replies so far.
There have been some updates since I last posted in trying to follow your suggestions...
In response to Sarang:
The fonts only render differently in Powerpoint. I tried several applications and it was only Powerpoint on the Macbook Pro that was rendering incorrectly.

Is the font embedded? Well, this is where it has got interesting (or not!).
I had to look up how to embed fonts on a Mac and found that I was using a version prior to that ability. So I have updated my iMac to the latest version and now the font is rendering incorrectly in Powerpoint on both machines! (The Macbook Pro would already
have been updated to the latest version as it was a clean install).
In response to Jim (and Tisky):
Yes, the fonts are definitely the same on both machines.
So it seems that the issue is either with the latest release of Powerpoint or with the font's compatibility with PowerPoint.
The font is a proprietary font to my client, so I am also going raise the issue with them to see if there have been any updates to the font or known issues.
Otherwise if anyone else knows of latest version of Powerpoint causing issues with fonts that previously worked fine and developed a work around, please let me know!
Many thanks
Lee