Adrian,
I agree that Mr. McConnell's work provides a wealth of information on English black powder artillery; I've been through it twice now, and it certainly does cover a lot of ground concerning the history (around 150 yrs worth) of British ordnance.
Douglas,
At an earlier time weren't you interested in the travelling carriages for large mortars used in the Boer War? See pp. 183-186. Fig. 141, 142, and 143.