Following a shout around my office, I can confirm that this is indeed true. A380 coming to do ground handling and airport compatibility assessment at LHR on 18 May.
I am told it's scheduled in at 12.00L (Midday) and will be going to T3 for "compatibility trials" - departs the next day sometime. Not a SIA a/c (although it may have it painted on the side) - this will be a flight test aircraft (not fully certificated yet) operating on French Reg under Airbus ownership.
Hope that helps!
RX
(p.s. I'm not a "spotter" - just work in the business for the UK Airline that has 6 A380's on order)