As far as I know it's a RS "owned" brand, so they are probably buying them as a OEM in china or where ever it's made and branded with RS-onlines instrumentation brand, in this case Iso-tech.
But knowing RS online, and where I work we buy from RS-online when we need to get various low quantity items on a day to day basis, but that service comes with a hefty premium price tag. So to be honest you can probably get a meter at half if not lower and get the same quality if you buy DMM else where, or if you the money to burn on it, look for other vendors who is alot cheper then RS and buy a agilent or fluke DMM.