Update.
It took three attempts to sort this.
With the cowl removed and the sensor ring edged outwards, the key worked every time.
First I removed all the overlapped leather on the inside of the cowl and reassembled. That wasn't enough.
So I ground away some of the plastic on the inside of the cowl around the hole and reassembled. That wasn't enough either.
So I ground most of the plastic around the hole to the diameter of the ring, so that the ring now sits just behind the leather trim. No plastic in the way at all............
I also replaced my replacement neoprene around the barrel with leather!!!!!
Hey presto......now I insert the key (without turning) and the immobiliser disarms in two seconds.....one full flash of the warning light, and a second short flash. So far over about half a dozen tests its been consistent............
So time will tell. It appears that age or use affects the coil and transducer and reduces the range. Given that we're talking mm's a tiny reduction in range is enough.
So if it happens when you're out, remember the 20 second rule.
Wipe the black plasic part of the key around the lock barrel. When the light goes out you have 20 seconds to start the engine.............
