Greetings Nick,
I had some success using the tag() expression function along with the TagPath (located in the “Meta” folder on the UDT property). The TagPath only navigates to the root of the UDT, so you’ll have to concatenate the tag name and EngHigh. Try the following:
tag({Tank Template.UDT::Meta.TagPath} + "/Temperature.PV.EngHigh")