Hi Andrew, Charles, thanks so much for your help.
I managed to work it out just by using "jason tab" as an extra google search term. Btw lol to the "jason" reference I found here Word: Getting rid of the ‘Jason tab’ in a TOC | CyberText Newsletter
Using that link I played around a bit and found out that it was indeed a rogue tab at 3cm at toc level 3.
So I did this to remove it and it worked a dream:
On the toc, right-click and choose edit field.
Select toc, select modify, select toc3, select modify, select format, select tabs, select 3cm, click clear, press "ok" four times, select "replace this table of contents", select ok.
Much appreciated :-)