Well I would argue that no groove in a mattress is supposed to be there, so it is not within industry standards. Also if the groove apeared (and you reported it) within six months, then you would be entitled to a refund, as it would be deemed to be faulty when manufactured.
You have to remember that the so called independent technician works for a company who earn a lot of money from Dreams. So just how independent do you think they would be??
As for Dreams (and any of the other big retailers), avoid them. With most of them you will pay at least double what you need to, as they have fake price promises, and fake sales. Also avoid most of the big manufacturers if you want a good mattress at a good price. Instead shop on line, but make sure the retailer gives a proper satisfaction guarantee, that allows you to unpack and sleep on the mattress.
Have a look at this mattress advice site: mattressadvice.co.uk
Regards
Tim