Although the solution provided by Kevlar is rather elegant, it maybe overkill.

I see from your profile that you are in the UK. The RU 980 can transmit a 433MHz RF signal ( From memory, I think this is the right frequency - can't seem to find the damn manual to check the specs at the moment!). If your screen operates on this frequency and you can find the remote codes ( try searching the database in the the original pronto and the pronto NG file areas - as a start) you should be able to make an RF device in your PCF that will control it.

There used to be a program called Codegen which could generate the Pronto hex codes for X10 and a couple of other RF formats - perhaps this may be needed to generate the appropriate hexcodes. I believe that this program was withdrawn from general circulation by its author but you may be able find it somewhere on the web.

HTH

Chris Ryan