OK so I know I've been a long time getting this sorted, I hang my head in shame
Anyway I eventually got 'round to taking it to the independant specialist last week and his computer diagnosed the fault as
Permanent fault. Catalytic converter downstream temperature sensor. Short circuit to positive or open circuit.
Interestingly their computer reads the code as P1419, not P1416 as mine said...
The guy showed me the sensor, although it's "downstream" it's actually pre-cat, you reach it down the back of the engine but I'd have to look again to see whether it's on the manifold or the downpipe. So after a short discussion on the reliability of diagnosing "sensor faults", coupled with the fact that it looked difficult to reach and was probably siezed solid I booked it in for this morning.
Here's the culprit
So that's £35.25 for diagnostics last week,
£104.19 
for the sensor and half an hours labour to fit (£23), = £162.44 inc vat, all for the pleasure of extinguishing a little light and stopping the car from beeping at me several times a day.
Still I'm so glad to have it sorted.
