Hi Darren,
Do not despair, you are just one of the many millions who have a little (or a lot of) trouble parting off.
The very quickie lesson I gave you was definitely not enough.
I am one of the very lucky ones who seems to have no trouble parting off. But even I have tried many different methods to make it just that little easier.
It was only a couple of months ago that I was given a parting tool that really revolutionized my way of thinking, and it is shown in this post.
http://madmodder.net/index.php?topic=1370.msg12873#msg12873There are many of the type you are talking about, and as you say, they all look to use the same insert, but each one is just that tiny bit different. All you can really do is to buy one from a retailer who stocks the tips as well.
These two might help you a little.
RDG has only just started to get these in, and as of yet, only has the 20mm shank size. I am waiting until they get the smaller sizes in before buying a 16mm one to fit my holders. I don't like dealing with Chronos, so that is why I haven't ordered one from there, but if you go down the page a bit, they have the full range in and the tips as well. It is up to you whether you go for those or not.
There are two types of holders on there, the ones at the top are limited in their depth of cut, as the one I have is. The bottom ones seem to be the more versatile, as the holder can be extended or retracted to whatever depth is required (within reason)
http://rdgtools.co.uk/acatalog/Parting_Systems__Indexable_.htmlhttp://www.chronos.ltd.uk/acatalog/Chronos_Catalogue_Parting_Tools_82.htmlIf you do go for this type, and you still have problems, it may be your technique, then it looks like you will be having another visitor to try to sort things out for you, and give you an in depth dose of parting off.
John