thanks for your helpful post Jon Jennings. I do have some control over php.ini so I increased the memory to 256mb which was the max, and increased the timeout values to 120s which is also max allowed.
For this backup we had selected just the posts from the last few years, 149 files totalling 19.82mb. The compression stage went very quickly, but it seems to hang on 'uploading to Dropbox'
have asked the host if they can help