This six-wheeled "moon buggy" has been in approximately this form in our Lego box since the 1980s. Is it from / based on an official Lego set or something my brother made up?
The distinctive parts of that build are 4288 Wheel Full Rubber Balloon with Axle Hole and 3787 Vehicle, Mudguard 2 x 4 with Arch Smooth (in old grey colour). They do appear in several classic space sets.
But a search for sets which contains both of these parts together tells us that your buggy is not an official set.