This gets more and more strange...
Our 3rd party support have now got back to me:
I have tested the line of code that you suggested and it actually does not resolve the issue. This is a known issue with precreated requests that VMware know about. http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2001422
During my testing I tested that it did not work first, Which it didn't. I then implemented the code and it worked. But, this was not because of the change but because it had run out of precreated requests and it works if you are not using them. You could set the precreated requests to 0 which would mean that the request screen takes a bit longer to load but it would copy the description across. Or you could use the workaround as provided by VMware in the link above. (Which I found quite funny to be honest)
They have told me that they are working on it and considering it for a fix in the future. Apologies that it is not a very satisfactory response.
I removed my customisation and changed this setting, but it no longer copied over the Description again. I put back my customisation and it started working again.
By-the-way, you can't just type in 0 in the "precreated requests" field in System Admin otherwise it puts it back to 1 by itself, but if you clear the field and save it, it then saves it as zero!
I have no idea what is going on, but I'm going to leave my customisation in as it seems to have fixed the issue for us. I'll see what happens when I try and upgrade us to 9.1.5 (which I hope to do very shortly) and test what happens in that.
I'll update this post when I've done this.