Thank you all for your comments. My server support team have come back with the following comment on seo.url.php:

While investigation your issue with the file "seo.url.php" I found that the file containing string "$gzip == 1 ? base64_encode". We have malware/virus scanner (LMD) on all our shared servers. LMD has inbuilt cleaner rules to attempt removal of malware injected strings , base64 and gzinflate(base64 injected malware as well as quarantine 100% infected/malware file but the file "seo.url.php" is genuine file. I have added file's path in the /usr/local/maldetect/ignore_paths list so that this file will not delete again in the future by the system scanner. I would request you to please edit this file rather than rename/reupload it if you wish to make any changes into it.

Regarding the product.php they say
"The issue caused due to mod_security rules restrictions which are as follows :-
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ModSecurity: Access denied with code 406 (phase 2). Invalid UTF-8 encoding: invalid byte value in character at ARGSroducts_description[1]. [offset "153"] [file "/usr/local/apache/conf/modsec2.user.conf"] [line "23"] [id "950801"] [msg "UTF8 Encoding Abuse Attack Attempt"] [severity "WARNING"] [hostname "billando.co.uk"] [uri "/xxx/xxxx/product.php"] [unique_id "TnHZV9XlepQAAGG@jw8AAAAf"]
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I have disabled the rules for th URI "/xxx/xxxx/product.php' , now the product can be Previewed ."

They seem to have been able to edit with define_page_editor.php but it is still throwing out errors for me.