On Sat, 9 Mar 2019 13:49:48 -0800 (PST), David
wrote:

On Saturday, 9 March 2019 18:25:13 UTC, John Rumm wrote:
There is a leak somewhere in my central heating system - its losing
pressure regularly, and needs topping up with at least a couple of
litres a day.

Any bright ideas?

I had a cracked heat exchanger. Because it was a condensing boiler, it happily drained all the leaked water down the condensate pipe.

Could you see whether the boiler appears to be producing condensate even when it's powered off?

Alternatively, can you isolate the boiler and see whether its pressure decreases over time?

I had a similar problem with a boiler with an aluminium heat exchanger
corroding severely over time (Only about 10 years). Found where the
problem was when one day it was all quiet in the kitchen and I heard
the trickling water out of the condensate drain when the boiler was
off. (I have some interesting photos of the dead heat exchanger if
anyone wants some teaching material for plumbers.)