2007 Vauxhall Astra Life estate - door lock trouble

I've been having some difficulty with locking and unlocking the doors on my Astra, but it has now become a serious problem.
First, it only ever happens when I am at home and not all the time either.
The car does not respond to the buttons being pressed on the key. (either key)
This is fine if I am getting in to go somewhere - can unlock mechanically. However, if I am leaving the car, it makes it difficult to lock at all and impossible to deadlock.
The car responds to the key in certain positions inside the car, but having done that, if I open the door to get out, it unlocks everything again.
I have found a rather strange method by which I can engage the first stage of the lock, so it is not completely hopeless.
However, this really isn't good enough. There does not seem to be an easy method to engage the lock if pressing the button on the key does not work from the outside, and certainly no way to engage the deadlock as far as I can see.
If anyone can advise me how I am supposed to lock the car in these circumstances, I would be very grateful. I suspect there is a source of radio interference causing a problem near my home, but unless (a) I can trace it and (b) it is caused by faulty equipment, there is not likely to be much I can do about it.
I am taking the car to have this problem investigated tomorrow to see if anything can be done to make it more reliable, but it would be helpful to know the emergency locking method, if there is one, anyway, and if there isn't, why there isn't.
Phil Reynolds
mail: snipped-for-privacy@tinsleyviaduct.com (though I'd prefer followups on group)
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