After much wailing and nashing of teeth, I've finally resolved this error which was preventing me adding large files to SVN. I tried many combinations of 'LimitReqestBody' and 'LimitXMLRequestBody' Apache directives as suggested by many Google search results without success. I belatedly checked the Apache error logs and discovered that the culprit was actually the 'mod_security' module. It was reporting 'ModSecurity: Request body (Content-Length) is larger than the configured limit (131072)'.
Upon examining the 'mod_security.conf' configuration file, I discovered that the value 'SecRequestBodyInMemoryLimit' was indeed set to the default 131072 bytes (128 KB). I commented out this line and the problem disappeared! I suspect it will now allow file sizes up to the 1GB hard limit. There is another parameter 'SecResponseBodyLimit' which may also need adjusting. This is set to 524 288 bytes (512 KB). "Warning: The information contained in this email and any attached files is confidential to BAE Systems Australia. If you are not the intended recipient, any use, disclosure or copying of this email or any attachments is expressly prohibited. If you have received this email in error, please notify us immediately. VIRUS: Every care has been taken to ensure this email and its attachments are virus free, however, any loss or damage incurred in using this email is not the sender's responsibility. It is your responsibility to ensure virus checks are completed before installing any data sent in this email to your computer."