Renault 11 GTL runs fine until I slow for a junction and then it wants to stall.Any ideas.?

Could be a few problems:

The main one I have in my mind is a problem with the charging system

Charging system - the alternator provides more charge to the battery when engine turns over quicker, this is why if your engine is idling and you turn on the headlights or air con, the car starts revving a bit higher. When you slow down at a junction less charge is being given to the battery so it is liable to stall.

Could be the battery is on it's way out.

There are other things I can think of, but you would tend to get those problems at low revs anyway e.g. when you start the car, or engine is just idling for instance a faulty idle control valve/sensor.

My advice is take your car to a garage. With a multimeter they can easily check the condition of battery and whether the alternator is charging properly, they are quick and easy checks and they should really do them for free.