First, thanks for all advice so far - unfortunately my own tinkering seems to make matters worse every time I touch any part of the site, so I would like to hand the problem over to an expert.

Here is an update on the issues since this project was posted. The version is 1.4.1.1 which has not been updated to any other version.

Initially I had the problem of a broken image zoom and a broken 'add to cart' feature, and assuming it was a javascript issue I replaced the js folder. This fixed the zoom problem but not ADD TO CART.

My main problem now is that when you click ADD TO CART it results in a 500 error in Chrome and IE, and just a blank screen in Safari. I ran the magento_cleanup script to fix all the folder and file permissions but it still didn't work.

In response to a request from a Maglancer, I have:

Turned on system and exception logging in admin
Renamed errors/local.xml.sample to local.xml
Retrieved var/log/system.log
Deleted folders in var
Verified the database details were correct

The site stopped working completely as a result.

I have now got it (half) working again after replacing the database and the app folder. Now, however, the system won't allow me add any new admin users - it says they already exist even though none are listed.

So here are the problems:

1 Add to cart goes to a 500 Internal Server Error
2 I can no longer add new users under
3 Whenever I flush the cache from within the admin area the site breaks and I have to rename the app folder to app-temp, rename a spare app-temp folder to app, refresh the site, change the names back again and start again.

I would like to select a bidding freelancer within the next couple of hours, so you are interested, please make a bid and I will supply all relevant access details.