Simon,
Martin Evans had a construction series for a Royal Scot some years ago in the ME - drawings on Blackgates web site.
.I think there were also some LBSC designs, but I think you have to be careful since he wasn't too particular about keeping the scale correct, especially with boiler size
You might also be interested in a book - "7mm Livesteam - The Eddie Cooke Articles" which is published by the Gauge 0 Guild. This book is a collection of articles on live steam locomotive construction in 7mm scale with a lot of dimensioned detail drawings throughout - but no GA drawings of complete locos.
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shewn as 'Live Steam Book' at £6.50 to members, but is priced at £7,50 to the rest.
Jim.