Brian, Whilst I agree the loco is not perfect, saying that it is not "fit for purpose" is probably a bit of an overstatement.
I don't know your age, but I'm old enough to remember when we paid a king's ransom to buy brass steam locos and then had to just about tear them down to correct defects, before they would run sweetly.
But, you know what, we did it because we wanted the locos to run properly and we knew that if we returned it to the importer there were a few possibilities:
1. We might (if we're lucky) get it back in 6 months
2. Even then there was no guarantee it would run as well as we'd like
3. We could roll-up our sleeves and do some "modeling" as opposed to "plonking".
Now, you've already done half the work.......you've identified the problem. That's great.
Now, try to imagine how much self-satisfaction you'll gain by FIXING that problem.
Here's a couple of clues or ideas in this particular instance.
1. As with MANY model steam locos, the front spring is clearly too long on your model. It is a simple task to remove the retaining screw, remove the spring and snip some off it. Looking at the photo, I'd say about half the length would be okay.
2. Place the spring back into position with the cut end facing upwards towards the smokebox. I'm assuming the other end of the spring is shaped such that it doesn't have an "open end"? Another alternative, if the original spring is too wide, try using the spring from an old retractable biro.....those springs are a narrower diameter and may, possibly, be a better alternative.
3. Find (or make) a small washer; one that has an inside diameter the same as the bogie pivot bolt. Place this washer between the bogie and the spring. It will (should!!) prevent the spring from working its way through the bogie's slotted hole.
The aim here is to have the spring just long enough to provide a MINIMAL amount of downward pressure on the bogie to prevent it lifting off the track and thus de-railing WITHOUT actually providing any upwards LIFT to the loco, thus robbing it of much-needed tractive capacity.
The same goes for the rear bogie.
If you feel like doing a bit more "modeling", why not add some small pieces of lead weight to the two bogies to aid them in their quest to stay out of the ballast?
So, getting back to my original statement, you have a couple of choices....you could whinge to the importer who may or may not be interested in your plight (he has your dough already so there's not much in it for him to go out of his way to keep you happy), or you can ....to use the good ol' Aussie vernacular.... "have a go" and feel the satisfaction drench your skin when you see it running sweetly.
If, per chance, you suffer from some incapacity that precludes you from doing this type of job, then maybe you could prevail upon a member of a model railway club to assist you or something similar. I'd be happy to assist you in that instance, but you probably don't live anywhere near me (I'm at Kadina, in country SA), so there will hopefully be better alternatives.
Cheers and good luck with this minor issue.