That is certainly a good buy.
I believe Iso-Tech is the old RS Components brand that they have now rebadged as RS Pro for a while range of products from test apparatus to hand tools and some components. Probably why it is being sold off at such a good price.
The RS Pro brand test apparatus seem to be a mixture of either Appa Technology or CEM designs. I believe the IDM66RT is an Appa Technology design, similar to their M series multimeters.
The link you have posted to in RS is for a UKAS calibrated device, which always puts the price up. As it stands without RSCal or UKAS, they sell it for £105.97 including VAT - £88.31 without VAT. Still a great buy though.
Kind regards.