You lose it if it's not used within a year of leaving. You can call the DVA, google them, and ask them if your clearance is still current. They can't tell you what level it is, but can cofirm if it's ok.
it may last 12 months, but you will find most new jobs will want you to do it all over again. I've been cleared to SC since I was 17, and every job I have taken has made me go through the forms again...even going TA to Regs I had to do it.
The only exception has been the current job where I managed to get the 2 security departments to talk to each other and transfer the clearance.
Just don't be surprised if you get asked to fill the forms in again.
They can't/shouldn't discriminate between someone who has the clearence and someone who hasn't. If you do have it though your more employable as you won't fail the check when they employe you. Company I work for aren't bothered, everyone who starts gets it.
I've just had to fill the forms in as mine is about to expire, otherwise its transfered by your company and the DVA.
Jimmy. Your new company will contact the security controller of your current company( or the company that holds your clearance ie EDS, GD etc if you are sub contractor) and the clearance is moved across. You don't need to do anything except let the new company know contact details for who currently holds your clearance.