OK, this is probably going to be one of those embarrassing, told you so, type of conversations
I got a Vivitar 2.2x Telephoto Converter lens (the type that screws onto the front of the main lens like a filter). Luckily it was cheap, and I didn't expect too much, because I think I got even less. The accessories it was bundled with are probably more valuable.
Or, maybe I don't know how to use it???
I put in on the front of the canon EFS 55-250mm kit lens, and there is a ring of out of focus area all around the centre, getting worse as you zoom, until, at 250mm, it is ALL blurry. It improves slightly if I stop it down, but even at f/32 it is still terrible.
Here's an example at 55mm
and at 100mm