Hi Derek! Welcome, by the way, but you might well be right in feeling a bit cheesed off!
As an old fan of Mantis, and because I've extolled the virtues of the kit for many years, maybe even helping others to make their minds up and get one, I've just been trying the links you mention, plus any emails from them. Pretty unhelpful aren't they! Some old links just don't work at all!
The last time they contacted me for any reason was December 2017 - they used to be in regular contact from 2013, (when I got a small one), trying to sell more stuff, and sometimes I took them up on something like a service etc. Nothing since!
It seems that Barrus aren't much more than agents for repair and service, but you already know this of course. As third parties, they also seem to do this for Wilkinson, Kawasaki and a couple more well-known names, so they must have got some agreement to actually do something, not palm you off as you have been!
This really isn't helpful, but as others may be in a similar position, it may just have to come down to you getting a credit card refund (?), or conducting a door-banging exercise on Barrus's website. I'd try both, but also, is a repair a possibility? Your blood pressure may go down a bit if you can at least get it mended! This site below seems to be working, but do you have a garden machinery firm nearby?
https://mantis.uk.com/mantis-tiller-owners-faq/Let us know how you get on - you shouldn't have this aggro after spending over the odds, chums here might be affected too and should be aware of all this, and I sympathise!
Best wishes!