New Caterham R420 Coolant Puddle under Expansion Tank

Most likely radiator failure.

The coolant damages the rad when it’s left unused for long periods.

I had 2 new rads in 18 months, Caterham didn’t want to know when I said the rads aren’t fit for purpose.

Quite common in the 420 I’m afraid. Hope it’s better news for you, if not its £600+ for a new one.

That's a very strange thing, how does it happen and what is it about leaving it unused that causes it? It's just strange that a radiator which contains coolant by design should be damaged like that.

So took nose cone off and had kitchen roll underneath last few days so marked the centre of the liquid and it was directly underneath the radiator reservoir tank and the radiator/oil cooker sits a good 6+ inches in front of that....asked my wife with smaller hands, better eyesight etc etc to feal under reservoir next to hose and yes there was liquid there around the hose and tbh hose wasn't sitting straight and jubilee clip not as tight as perhaps should be..so reset pipe, tightened jubilee and put some sealant around just in case....

I have had issues with radiators before on other cars when have not cleaned winter salt off them properly and then put car away...had forgotten but these messages reminded me to be a little more mindful and yes good ideas will change 'orange' to 'blue'!!!

As always thanks you always helps knowing folks are out there to help :-)