Thanks for the quick responses guys. Unfortunately I think in an all-time first I really wish you hadn’t, as as soon as I submitted the post I scanned through my code again and realised that a closing bracket was missing from the plugin tag. I could’ve sworn that I put one in there after the 50 item field list but apparently not! Anyway, no chance to delete my post so I’ll have to admit to it. 
I’m now in the process of importing nearly 22,000 entries, each with around 50 fields. So far the plugin has been faultless and I can’t thank you enough Andrew for providing such a time saving tool. One change I have had to make which may be useful to others, is that the duplicate title checking was a problem for me as I’m importing a huge list of businesses, some of which do have the same name. As a result the standard behaviour of the plugin was ignoring 6 out of the 7 businesses named ‘7 Eleven’ for instance.
To disable this just change the following on around line 148:
if ($query->num_rows == 0) {
To:
if ($query->num_rows == 0 or $query->num_rows > 0) {
(Yes you could also remove the if statement entirely, but this is a little easier than digging around for the closing bracket as well, and we all know what trouble I have with them.)
Also if anyone else has so much data they need to split it across several files, here’s a semi-batch import solution which may be of use:
<?php
print "<h1>IMPORTING DATA</h1>";
ob_flush();
flush();
$nextpage = "{segment_3}" + 1;
?>
{exp:csvgrab
url="http://yoursite.com/data/{segment_3}.txt"
otherparams
}
<br /><br /><a href="{path=import_debug/csvgrab/<?=$nextpage;?>}">Next File</a>
Finally, again if you need to split a large file, I used GSplit which is free and lets to specify exactly how many lines you want per file.
Hopefully that’s all of use to others if they need to do something similar in the future.
P.S. Small world, David. I’m in Portsmouth and have been to a fair chunk of the venues on your photos site. Nice site it is too btw!