Quick check, is it 'sweet' or plain water (not more than a fingertip touch as coolant is toxic) and are you loosing coolant?
If not loosing coolant, the cabin filter drain is likely to be blocked as said; remove wipers, scuttle and take the top off the box, lift out soggy filter and clear leaves and cr@p out the bottom and then driver footwell side panel and clear the revealed drain tube.
Replacement parts available from local motor factors for a fraction of OEM cost, (list is in alternative parts thread) I use the Cooper one £8 carbon charged.
If sweet/confirmed coolant loss, I'd be looking at a split hose or fractured pipe as the likely culprit.