It wasn't too troublesome, really!

My conclusion, for what it's worth, as to why the transmitter went out of alignment...
The Veto works on a 'rolling code' system, which means every time the button is pressed the code changes slightly. A very slight difference in the code will be assumed by the alarm to be just because the button was pressed while out of range. If the button gets pressed too many times while out of range then the code will have changed too much and no longer be accepted by the alarm.

The transmitter will still be recognised as being 'paired' with the alarm so will accept being re-aligned. Obviously it can't be done with a new transmitter which has never been initially paired!

I think that's how it works!

Thanks Nick.

Good to know. Luckely it happened at home and so I was able to use my spare fob. Had I been away from home and not known it could have been inconvient, to say the least.