Hi Richard,
I checked the book "Paddle Steamers" by Richard Clammer, this is on the inside cover....it says the plan is in the authors collection, so he must have got it somewhere. Its quite an old book, reprinted 1983.
( I'm sure a mod. can remove this scan if there are any issues with it being posted here?)
I see the Strathmore was bought by the Admiralty in 1908, maybe there are plans under HMS Harlequin somewhere.