There are times when I want to leave the bonnet open, but lock the car (specifically now to check the float volts on the battery periodically as I think the battery is shot). However, it refuses to lock.

I am guessing using the manual key may be the only way?

Have you tried pushing the two bonnet catches down manually .i done it with an l shaped ,bent screwdriver .

I have thought about that, but not done it - could not see where the sensor wa.

When looking from the front with the bonnet open, the switch is in the left lock. Use a torch and look inside the hole where the lock for bonnet is and you will see it.
I had to replace mine as it was faulty.

If it's just to let a cable out, you can pass it down and under the front bumper. Just remember to remove before you start the car! (Place a note on the steering wheel. A large note). Ex AA Series III LWB Safari - Gone
300TDi Disco (bought new - terrible car) sent back after 18 months
Freelander 1 Estate - leased, given back at end of lease
200TDi Disco (bought from a mate with 100,000 on the clock) - Gone
Disco 2 TD5 - sold and exported to France
FR2 TD4 GS - Gone
FR2 SD4 HSE - Now changed for a DS
New model ex-demo Evoque S 180 in white
Unable to order a new DS, so gave up. Now have a Volvo S90 Recharge.