Target Display Mode - Black Screen

Hi

I am trying to make the following configuration work, as per TDM details from Apple (https://support.apple.com/en-gb/HT204592):

MacBook Pro 15” (Early 2011) —> Mini Display Port Cable —> iMac 27” (Late 2009)

Apple F2 works - the MBP is seeing the iMac as a display - I can configure resolution etc when I gather windows on the MBP - but the iMac just shows a black screen.

• Both machines are running 10.10.3 with all updates applied.
• The iMac has a wired Apple keyboard
• I’ve reset the SMC and PRAM on the iMac
• I’ve tried the boot order as described in this thread: https://discussions.apple.com/thread/4215736

Would really appreciate any further ideas from the community or from Apple on this please?

Many thanks

Andy