If i was working a LARS and the weather deteorated at my destination aerodrome and wanted to divert to my alternate how i would i express this?
G-CD diverting to Odiham
You don't call him Farnborough, he knows who he is and you have already established communication. He will probably tell you to contact Odiham.
Would i ask to transfer to my initial destinations freq and tell them i am going to divert or will the LARS do that for me?
If you are not talking to them, how would they know you are coming?
Also when i change to the diversion aerodrome they wont know im coming on 1st call so in rt speak how do i ask to land with them?
When you call them you commence your call with your request. G-ABCD request weather diversion to Odiham. From that they will know you want to land there.
Sounds like you need some instruction before going anywhere near an RT Test.