Hi Stav
Do you have the RAF number of the launch? It would be either a three or four digit number on the bow usually something like 358 or 1664. This could help find the exact type of plans you're after due to different types of 41.5ft boats used by the RAF- there are actually a lot of sub variants of the Seaplane Tender with subtle differences. If you do, I can possibly trace the history of that particular boat if its in my directory collection of the RAF Marine Craft.
I have found that John Pritchard drew up some plans of the 41.5ft and published in Model Boats magazine in the 1980s:
41.5ft Seaplane Tender Mk 1/1A Profile and G.A. Plan No: MM139
I thought I had a copy of these plans but I was shocked that I haven't! Very unusual for me for not having the 41.5ft plans as I have the 37.5ft Seaplane Tender plans- must sort out my plans in an orderly system from now on!