Hi ya Gra
You must be building an early version of the Fairmile D. As the early boats were the only ones with the Hotchkiss x 6 lb gun mounted on the rear of the vessel. It was very soon upgraded.
If you do a search for the Fairmile B (as this weapon was used frequently on the front of these vessels) throughout the war - so, you may find a few more pictures of the weapon in use on the Fairmile B.
If you go to the library and see if you can source John Lambert's book 'Allied Coastal Forces of WWII - Volume 1' there is a good amount of information in that book.
http://www.dockmuseum.org.uk/archive/browser.asp?subject=Armaments&title=Naval+Weapon&subtitle=Naval+Gun&searchtype=1Have a look at the above link for Dock Museum in Barrow-in-Furness and this link takes you to the weapons site. There are a few pictures there of the weapon - but, sadly not that clear. Have a hunt through (mind there are hundreds of photographs)

you may just find a few clear ones. I am sure I have a drawing somewhere so I will have a dig round.....
aye
john e
bluebird