Isn't Didcot a loans act shed if so it well photographed and I'm sure there are plenty of drawings in books. It has the advantage that it's still standing to so you can pop along and get more details.
As to how to make one depends on the scale you intend to do and how acurate you want to detail it. The other thing is how much you want to spend. Timber tracks do a couple of engine sheds in both 4 and 7mm I don't think they are loans act ones though. To get correct coursing of your brickwork it's very hard to do with plastikard though but a good representation can be made.