MM wrote:
There's a white plastic pipe between the oil tank and the boiler. I
want to clip on a flow meter to measure the oil actually being
consumed at any one moment. Is there such a device?

Dunno about that but
http://www.canongatetechnology.co.uk...ckst.html#prod is a range of
products for non-invasive tank level measurements based on ultrasonics.
Maybe you'll find something there, or maybe not.

John