Troubleshooting a Blank page at survey submission

My user has created a rather large survey at 300 questions and 35 pages :ohmy: They are getting to the end of the survey and finalising it , they then get rewarded by a fairly long wait and a blank screen. The responses are all logged in response_details thank goodness and the survey has a closing message which is not shown.

I turned on system debugging, CS debugging and full messaging and yet I get a blank screen with view source only showing the header and body tags

I have upgraded my server to allow for 199MB of memory and pushed the PHP limit to the max my ISP will allow. It does have the feel of a memory issue however without the console showing I have no way of knowing.

I was thinking of trying an alternative URL to the closing message

Other surveys are running properly although they are of course much smaller.