after a bit of looking around on the internet I came across don's, G3BJ's page. he has a schematic for such a device on his web page G3BJ RX antenna protector along with some other designs for various other radio related stuff.
I did some shopping online for parts from here Bitsbox and now time to put it together.
I used parts as described in dons schematic.
I first layed out the parts in the box to decide where the various parts would go. Relay is a RTE20412. 12V.
once I had the layout I was happy with, i soldered everything in. below, all electrical soldered.
Below, with RF lines connected, testing with 12v applied and the green phono is supplying 5V's, as if it was being keyed, PTT. I used a multimeter in continuity mode (BNC's) to check the relay was switching when 5v's is applied (green phono).
All was good under testing, so finished boxing it up.
thanks don G3BJ