can anybody enlighten me as to why, in a domestic premises an engineer would set a ten second delay between alarm trigger and outside bell box activation.

To give you time to enter the code and disable the alarm?

would this be standard practise?

Remember that during that period if the alarm activated the internals would sound, then if the correct code hasnt been entered, then a full alarm activation would take place. More often found in part set modes at night so if the alarm is nuscence tripped in the morning by the house owner, they can switch it off before the whole street is woken up.

would this be standard practise?

It would be something discussed with the house owner, and if applicable insurance company. If the alarm was monitored it would be a different set-up again, so it depends on circumstance. If the said person is unhappy with the config, then get it altered if it is a worry.