I've built one of these before. The kit is a Mercian etched brass product in 4mm scale (OO) for an ex-LMS dock tank.
More years than I care to think about have passed since I made my model and in the intervening time my skills have increased as has the toolkit available to do the job. This is good news as the kit is best described as "challenging" (in light of the recent comments by the culture secretary I better not put the words I used at the time in here for fear of an X rating for this blog !). In fact my version included a significant amount of scratchbuilding - boiler (I couldn't roll them at that stage), firebox, smokebox and cab roof. This time I expect to be able to use all of the kit. At least building the Garratt has given me plenty of practise making up Walschaerts valve gear !
Having said this, the resulting loco looks fantastic and works pretty well too. It's always been one of my favourite models and hopefully this one will look as good, if not better.
For more detials on the prototype, try Wikipedia