Thank you all very for your responses: you have given me much to think about, and have helped me - start - to crystallise the issue.
My problem is that I'm allowing the pulse to "breathe" too much. If I were playing with another musician, they would be intensely irritated... I might not beat myself up, but they would! Far too often I lag before a beat. And I unconsciously adjust my counting to suit.
I think it's necessary to try to pinpoint what exactly the problem is in order to be able to improve efficiently
is probably the key.
I'm wondering if I get into bad habits when learning pieces, slowing down before slightly trickier chords / passages and then ingraining that into my fingers / ear. In that case using the metronome as I learn, immediately after the first read-through, would probably help.
But I would still like to learn to stop that unconscious adjustment to my counting. I'd like to play with others and survive!
I'm not "rhythm dead" but it's certainly not my strong suit. When I doodle, I enjoy a feeling of painting with sound, but that sound has no architecture. I'd like to acquire that.
Any further thoughts are hugely welcome.