Reading NELPG's North Eastern Revival booklet from the 1980's, they state the tender of the Q6 was becoming full of tools and spares so they bought a 10ton box van to store these in, not acquired another tender. As far as I know, it came with the tender it was withdrawn from BR with. The J27 was the loco they did a tender swap with I think at that time. They did acquire another tender some time later, but this was a group standard one, so its better condition wheels could be swapped with those under the K1 tender. I believe the frames of this tender lurk somewhere around Grosmont.
The Q6 tender was the subject of a Manpower Services scheme in the mid to late 80's when NELPG had a base at ICI Wilton, when the chassis and running gear were fully overhauled and a new tank constructed. Presume its self trimming, but you could always ask a mutual friend who would know!