Thanks for all the comments, suggestions and advice. Just to be clear, it was absolutely not my intention to 'start a scare story', I was just wondering if anyone else may have experienced the same issues possibly with a view to finding a solution. If the moderators feel my comments are inappropriate then feel free to delete the thread.
However, if anyone is interested (and in reply to some of the other comments and suggestions), I have removed the chains completely, mainly to facilitate the removal of the bogey frames. Chains can be put back on at a later date. All wheel back to backs have been checked and tweaked. Some of the underframe detail on the tanks has been delicately trimmed back as this seemed to be fouling the bogey rotation ( not visually noticeable either). I have also trimmed a small amount of the black plastic flange that is located between the wheels and axles. The wheels now have a little more play side to side which seems to have eliminated the problems and it is now running nicely on nearly all of my layout. I will also trying loosening off the bogey screws by a quarter turn as suggested. Maybe I just got a slightly off one? Feel free to have a look at my railway on Halton Road Model Railway on YouTube, there are couple of videos of the whole layout on there. I'm sure you will be able to see why I was perplexed by its performance.
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