The remote opening of doors can fail sometimes due to unexplained interference and/or low battery in the remote...the battery power can sometimes be ascertained by the distance from the car one needs to stand to make it operate the central locking. Batteries are fairly easy to change and aren't too expensive.
This has occasionally happened to me and with the spare key too....usually works if you stand right next to the driver's door and point it at the module.
Once it has worked it seems then to be OK...go figure.
But saying all that I wouldn't change the Brant for quids...the peace of mind knowing that the car can't be started without the code far outweighs slight annoyances.
It may also be worthwhile to reset the Brant if all else fails.

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