In summary.
Go to....

http://www.conceppt.com

Look at the menu column on the right and click "Downloads 2"
Once there scroll down the page and click "Conversion Service"
A new page will open. Click the "Choose file" button and upload your file.
A bit further down you'll find an "output format" menu/button: choose format and then click the "OK" button.
Wait for a minute whilst it converts and then download the converted file.

I suggest that before uploading you use QuickTime and "export" your video to a .mov file, with the following settings..
Video...Compression: H.264, Quality: Medium, Frame rate: 15, Encoding mode: multipass, Dimension: 160x120, Sale: Fit within (160x120).
Sound... Format: Integer (Big Endian), Sample rate: 44,100 khz, Sample size: 16-bit, Channels: Mono.
I've found that all .flv files, of whatever size, converted as above, work. Therefore if youre not having any luck convert first to .flv and then use QT to export to .mov, as above and then upload to the convertion service.

Cumbersome but effective.

Ciao, Wamba.

Wow this thread has some serious page views happening!