Having just read all this, thought I'd give a little ATC insight although I'm not linked to either Cardiff or LHR.
When an aircraft says it has ANY problem, we think a little different about it no matter what has happenned, due to the 'what if' factor. Caution has to be given generously, and we would rather be safe than nieve. In this case I would expect the Tower controller to put on a 'local standby' where all emergency services are ready to go, but not required to convene at the RVP points in the airport. The Heathrow Fire Service may decide to greet the aircraft, thats their perogitive. And if fluid was definately dripping visibly, the runway on which the aircraft landed would most certainly have to be temporarily shut for a full surface check, so in our respect, it is a kind of 'emergency' even if not directly life threatening, and our workload obviously increases.