Toyota Avensis Central unlocking will only work when very hot outside

I have an Avensis 11. The central locking works every time, but unlocking doesn’t

It only unlocks the boot and turns the alarm off. Then I have to use the key to unlock the driver door, then the 3 other doors need unlocking manually.

UNLESS the day has been lovely and warm, or I have driven for about an hour then it will work ONCE and once only.

I took it to the Toyota garage and they told me I needed a new door lock, which had zero effect. My local dealer is about an hours drive away, so uses up a whole day going to them as I have to sit and wait for them to finish.

What could it be?