andrew wrote:

I'm looking for the simplest monostable to provide a short pulse to
momentarily close a contact on our electric gate at work. Normally
it's opened with remote control but for visitors during office hours
is was equipped with a bell push for entry and exit, this being
cheaper than induction loops etc.

Some wag has found that if the outside button is depressed and held
down the gate cycles through open to closed and then will not accept
input from the other button or the remotes.

I'm after a simple way to have the bell pushes close the contacts
momentarily and then release even if the bell push is held down.

I've got a Crouzet MUR1 that I can get to do the job but it's about
30 quid and I wonder about something simpler?

If you are into electronics, a 555 can do that fine.

There's an example in the app note.