Very interesting thread. I bought a DCC fitted BR early crest 30673. It was a very poor runner. I did some tests and there was no electrical continuity from one side of the rear bogie. I removed the bogie (which is very easy) and one of the fixed contacts inside the loco was "springy", the other totally flat. I'm sure the bogie had been lifted slightly and checking the driving wheels carefully, I reckon the rearmost driving wheels were not quite touching the track. Now.....thinking this was the ride height issue, I did the Dapol "tweak" but of course, too much bending on the electrical prong from the bogie means that they never touch the internal contacts and definitely not the flattened contact.
Anyway, after confirming this was a fault due to the flattened internal contact, Rails kindly replaced the loco with a new one. The new one runs better and there's no ride height issue. It looks like slightly earlier M7 suffered from a pronounced ride height issue but the Dapol DIY fix seems a bodge to me. The internal contacts need to be springy and the prongs from the bogie need to be just right so once the model is placed on the track, the weight pushes the bogie onto the loco frame / body and the electrical pick up should be OK (allowing the spring of the internal contacts to sit on the prongs).
Based on the experience of the first model, if the prongs from the bogie are straight, its obvious that they are likely to push against the internal contacts and potentially flatten them. Similarly, they might force the bogie way from the loco body and hence push the back of the footplate up causing the ride height issue but I'm not convinced that's the whole story. If the internal contacts do get flattened then they are unlikely to give electrical continuity. There has to be some flexibility here.
Anyway, my replacement loco sits level on the tracks and seem to be OK. The bogie has some "spring" to it due to the fixed contacts inside the model but once on the track with the bogie in touch with the frame it looks fine.
The only other "moan" is the factory fitted decoder - its poor and no messing with CVs will help the abrupt starting and stopping so I've actually replaced it with a Zimo MN170F. Very fiddly to solder the smaller diameter decoder wires to the existing wires in the loco but the difference is worth every penny - superb low speed performance. Its transformed the model. Highly recommended.
Andy