I'm not in the UK myself (US here) so the rules are a bit different, but I did get a refund for this unit some time ago due to the other substantial damages it suffered. Many of the units, though, are shipped here from the UK (as this one was). I was actually going to scrap it since it wasn't wanted back (goes to say something itself), which now looks quite necessary since it is making a mess!
The place I've been talking with now also does commercial units and has rave reviews from restaurants. I shouldn't have any shipping problems, as I'll likely be picking the new one up myself. They are considerably better built (and therefore a lot more $$$$$), but apparently my story isn't uncommon and the quality on some of the low-end India-built units has too much variation depending on where they come from.
Their recommendation for Naan is to use a bit less heat, around 450F or 232C, but that could be related to the type of clay being used in these specific units. There is definitely a trick to learning it, I was told.
At least here, most restaurants are using gas due to regulations and greater difficulty of using solid fuels indoors - but I've been to at least one place that does use charcoal for the extra flavor.