At this stage I can't say for sure, but only one Mk.3 tipper was made by the factory, so if that is 'the one' then it is a rare piece of automotive history.
This is a photo of the unit that i'm talking about. I've not yet gotten far enough through the Mk.3 ARN's to find out what your rego number was to see if it is the same vehicle yet.
it does look very very similar however. definitely restore it!