I actually got through on the phone to the CAA licencing technical dept. today, it wasn't a very productive call. The lady I spoke to said "If your FAA declaration letter says it expires on the 8th April 2019 then thats when it expires and you'll need to re-apply again"
I said well, what if the BASA agreement is not implemented by the 8th April 2019, will my current FAA declaration be automatictly re-newed ? or will I need to begin the whole SRG2140 validation process again ?
At that question she gave out a long sigh and gave up second guessing what the answer may be and told me to direct any questions I have to the licencing team by email....thank you and goodbye !
Not the end of the world I suppose, I'll probably just have to bite the bullet and spend a few £££ by going the Part-FCL conversion route.