At the point of me crossing the upwind end of the runway and joining crosswind, I heard another aircraft call downwind (LH circuit). I couldn't see them but they were probably to my right
No, the other aircraft should have been on your left as the downwind call should not be made until abeam the threshold (CAP 413). One of the most important things when joining a circuit is to listen for other traffic on the radio so that you know what is going on from an early stage.
How much deviation in the published circuit is acceptable? Thoughts?!!
Apart from noise abatement the circuit is either LH or RH and its up to the pilot to position the aircraft based upon its speed and size. The concept that its a fixed line on the ground is nonsense.
As it was, climbing back to the overhead was not the wrong thing to do, but there could have been other traffic joining or departing from the overhead so it may not have been any safer than continuing your circuit with a good lookout.
The sooner that UK airfields go over to USA style 45 degree joins
Rubbish!