I basically started again, this time seemingly with more success, and this is a summary of what happened:
- copied all v2.76 web files and database from old server with Apache v2.0.52, PHP v5.03 and MySQL v.4.1.8 to Apache v2.2.9, PHP 5.2.11 and MySQL v5.0.51
- running upgrade, advised to add zend_optimizerplus.dups_fix=1 to php.ini. Done.

- re-run upgrade: Checking ... Disabling the forum ... Beginning decompress ... File Upgrade Complete ... All changed files were backed up to ... Convert table ...
- get to fud26_users, error "Delete from fud26_users where alias = 'I'm here'
- edited upgrade.php script to replace the data string ' with ''
- re-run upgrade. get to fud26_users, get "Duplicate entry 'xxx' in key 4.
- data actually seems fine, and there are seemingly no duplicates, a bit strange, so I dropped the index (alias).
- run upgrade again, get:
Notice: Use of undefined constant __dbtype__ - assumed '__dbtype__' in /var/vh/web/fudforum/upgrade.php on line 1094
Fatal error: Call to undefined function q_singleval() in /var/vh/web/fudforum/upgrade.php on line 1110
- restarted Apache, re-run upgrade:
- get to fud26_search, get "Duplicate entry 'xxx' for key 2
- again data doesn't appear to have duplicate entries for text stated, so I decide to delete all records from fud26_search
- re-run upgrade, it gets to the end.

Today I could get into the admin panel. Yesterday I couldn't get into the admin panel apart from once (where I got Fatal out of disk space while compacting private messages). Subsequent efforts resulted in the function errors mentioned in the previous post.

Today it has run through the consistency check, compacting and rebuilding of themes okay, and subject to confirmation from moderators it may well be working.

I've basically followed the same procedure each time, so I'm a bit concerned as to why it went wrong several times, and now seems okay.

By the way, all the UPGRADE_STATUS file contained after every attempt was one line with "4020".

One further thing, to get any of the above to work on either day I had to set all permissions to 777 throughout the site. Could you please advise what these should ideally be? And if you can shed any light on what I've posted, it would be much appreciated for all.