Have a look here David Geen ...
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he does a shed load of wagon kits, found him to be very helpful and for a one man show the delivery is very quick (haven't tried his carriage kits). The only kit I've found to be in any way fiddly is the cattle waggon - the bottom rails are a brass etch that has to be slid into place - there's a knack to it. I know folks can prefer brass/plastic because you can get a little closer to scale but for my money white metal has more of a "presence", the finished articles actually ridde the rails instead of bouncing around and have a sense of inertia and momentum.
Your Tot's a braver chap than I - the idea of a shed load of strangers peering over your work looking to find fault ... yuch