I spent many, many stressful hours assisting the owners of these locos, and rebuilt a few so they would actually do something akin to their advertised attributes. This was in 2010-2011 and it still haunts me to this day. A lot of good people lost a lot of money on these, including a major supplier of components. I do not have a list of serial numbers but seem to remember it was around number 25 that the "switch" occurred. Over the past 15 years I have done my best to wipe memories of these monstrosities from my mind.
You have said elsewhere that your one has been rebuilt. That can only be good news and it certainly will not make it any worse than it was. Major issues were the axle "pump" (it was never a pump), smokebox, bodywork and paintwork. Later ones had boilers that incorporated so much metal they were impossible to ever get hot. One of my rebuilds failed because of this.
I have many photos from back in those dark days.
I hope that you witnessed your loco hauling a train before parting with your hard-earned. One trick used back in 2010 when customers collected their loco was to use the known-to-work "shop" loco as a training demonstrator rather than their own, for such reasons as "we wouldn't want to get it dirty, would we?".
I refer you to
experienced driver, newbie fireman,
Sabre Steam Annette as Overhauled and with Short Blast Pipe and
Sabre Steam