Debug files not being deleted via admin

I have recently updated a site, plus transferred it to a new server (same host but different account on server) with a different domain name.

on the old server the cache file had 160mb of error files, I couldn't get them all to delete via the admin purge cache (most of them seemed to but a lot of really old ones didn't, and I ran the script a couple of times to ensure it wasn't timing out), so I manually deleted them, I also fixed the issue that was causing the errors.

Fast forward to the newserver, i did a completely new install of zencart (uploaded all files, set permissions on the folders as directed by the install script), copied across the template and database from the old site ran it and everything ran perfectly, checked the cache file, and there were errors, (i had forgotten to run the fix_cache_key fix after transferring the site), ran that and the errors seem to have stopped completely

Cool went to purge the errors via admin, it said they were successfully purged, went to check and they were all there still.

so my question is

1. what have I missed
2. how can I generate a couple of new errors, just to see if the new errors will delete
3. Yes I know I can manually delete the cache files, but i would like to fix the issue

Your problem with deleting them is probably permissions/ownership-related ... especially if the files you can't delete were files you transferred to the server during the move, and especially if you did the transfer by zipping the files and unzipping them on the server, since that process retains ownership and permissions settings, and thus can often end up causing problems such as unable to access or delete or sometimes even read them.

Whatever the "cause", it's a permissions/ownership issue that will need to be sorted out on the server. Zen Cart can only delete files that PHP has permission to delete, and that's all up to how the webserver administrator has built and configured those systems to work on that unique server.
As you say, you can delete the files manually, and that may be the end solution in your case, depending how cooperative your server administrator is with helping you.

No files were zipped up and transferred, it was a complete new install from zencart.com that i unzipped on my local computer, then uploaded. the only files transferred across from the old site were the images and template files.

I agree its a permissions/ownership error, but I can't figure out why, as the cache folder was uploaded at the same time as every other folder. and they all seem to work fine

If the files with which you're having problems were created on the same server where you're having problems, and were created using PHP but cannot be deleted using PHP, then you need to get your hosting company to help you sort out what's preventing PHP from being able to delete them.