Hi Stevie,
I think you are going to need a lot of luck.
We tow an Eriba Triton, run the Eriba Amiga web-site, and like most things with a bit of style you need to be prepared to look far and wide. So when someone asks me where they can find a used Eriba - then Holland is much more likely as a source than the UK.
Many Eriba owners have bought their (new or pre-owned) Eriba in Holland, saved a fist full of money and had a holiday as well.
Likewise for the Tabbert T@B XL - I work in Rotterdam and checked with the local Eriba and Tabbert dealer in Heinenoord (www.Campoworld.com) and they don't have any pre-owned XL for sale.
OK, next choice is www.marktplaats.nl - which is a free advertisement service in Holland - around 200+ Eriba shown but no T@B XL at all
And looking at the Tabbert factory web-site - the XL has been dropped from the model guide - leaving the original T@B and the T@B Off-Road - so if this is right, then production only lasted a few years and you may have more than a bit of fun finding one.
Robert