The callsign to set the alert for will be an SIA prefix, not SQ.
So, Alert settings should be:
Flight ID
SIA308
SIA319
Aircraft
A38*
And maybe add SIA* to the fleet watch list.
I can't remember what the registration is, but you could put this in the Registration alert box too! This should ensure that you capture the A380 whatever data it is showing.
Don't forget to tick the "Activate Alerts for Network Flights" box in the Alert section, this way you should pick it up way before it gets to Heathrow :-)
I don't have access to Network data at the moment, but maybe somebody can see the aircraft right now?