A litre in 70 miles is hardly using a little :shock: - you definitely have a leak that needs fixing straight away.
A lower pressure cap will let the coolant boil at a lower temperature than normal but, if that's your problem, you would expect to hear it boiling and see it leaking.

Originally when ...

It it advisable to use the proper radiator / header tank caps .. as like a kitchen pressure cooker .. it raises the boiling temperature of the water ..

Saying that I lost the one off the (plastic) header tank .. as I only had a 0.9 bars to hand (from motor bikes) so I fitted that .. always meaning ...

I've just bought my first Bongo (Bertie) and we have had a few overheating problems. I told the garage I thought the water was low and they topped it up for us so as we are driving home and she spat out her water and got a bit hot. We got this sorted and she hasn't overheated again but we are losing ...