Thanks for your reply Ralf.
The confusion over the two buttons comes, i think, from early attempts when you had to 'lock down the frequency' before transmitting?...something like that. Anyway, MCE asks to assign a key for transmission on the joystick at set up, am now able to talk to clearance delivery as you suggested, by informing FO that the radios are mine. So, i request IFR clearance to Dublin " 'clearance delivery Easy 1000 is with you requesting IFR clearance to echo India delta whiskey". CD responds, in this case clears me to 5000, squawk 2652 departure on 122.6. I read back the the clearance but there is no reply informing me that the read back is correct in fact, thats the end of all communication with ATC and the only way to start again is to disconnect PFE3 and then reconnect it.
Thanks for your patience.