Problem sorted with a new AC power adaptor. Unfortunately I ended up purchasing a replacement sling box solo as I thought my original slingbox was toast when could no get it to hard reset and only 1 red light was showing (after 5 mins connected to original power supply). Put a voltmeter on the original P/S and all seemed well and I tried other power supplies that I had in house before ordering a new slingbox but they did nothing - suspect that slingbox is very dependent on getting the correct dc power input.
In short problem traced to a faulty (for what ever reason) to a dc power supply. Buying with a new slingbox solo makes for an expensive replacement P/S hence lesson learnt so hopefully this note will avoid others making the same mistake.