Leaking coolant from rear

HI, I’m driving a Series 2 Elise ST, it’s approx 18mths old, and I’ve noticed that she’s leaking coolant from the rear. There are water marks up the diffuser below the exhaust system. I’ve kept an eye on the coolant level which goes down, but slowly and the temperature gauge which doesn’t give any cause for alarms.

Does anyone konw of any common issues that may cause this? or simply a loose clip? somewhere?

Look forward to any thoughts.