It's about two years old, non-condensing, and recently is taking a
long time to come up with hot water. It fires up about five or six
seconds after opening the tap, then it's anything up to about five
minutes before the water gets warm, not counting the time it takes
to get to the tap. The more recently it has been used, the less
time it takes. It doesn't seem to depend on the water flow rate.
The red light is on the whole time, but it's difficult to tell from
the sound how much gas it's burning. The CH pipes stay cold.

Does that ring any bells?