My 53 plate Zafira keeps stalling when slowing down at junctions, roundabouts etc. What would this indicate?

This sounds like the EGR valve, a common issue with some Zafira's (you don't say whether yours is petrol or diesel).

I own a 51 plate Zafira A and yet this problem has been common throughout the range. My car would do exactly as you say, when slowing down to exit a motorway for example, as I would slow to a stop, as soon as I engaged the clutch, the revs dropped and the engine stalled.

A temporary fix is to identify the EGR (usually at the top of the engine to the right of the valve cover or somewhere near the throttle housing) and simply unplug the electrics to it, wait a few minutes then plug it back in, restart the car. This seems to restart the EGR and reset it. This is however only a stopgap.

I found that this issue was affected by many things, the throttle housing for example, can get very mucky and if so the stall effect is more likely. If you have torx/star sockets the throttle housing can be removed after taking off the air intake pipe and cleaned with carb cleaner. The throttle assembly has small breather holes (pin size) which get blocked and can cause poor running also. Keeping plenty of oil in the engine (we all know how much these engines use) seems to affect this too.

SOLUTION!
Finally I decided to try the option which was suggested on other sites, which is to block off the EGR valve completely. You see the valve is there to reduce emissions when the vehicle is commisioned but does not affect the regular MOT. I ordered a blanking plate from ebay (Â 4) got a set of torx/star sockets and removed the EGR valve, making sure the gasket was left in place, and placed the blanking plate over the hole which the EGR covered. The EGR attached with two torx bolts, I replaced one of these to secure one side of the blanking plate, then the EGR valve itself is attached to the other side of the plate, with the base of the valve swung out. Then the second bolt was placed holding bothe the old EGR valve and the blanking plate. The reason for this is that the EGR will be recognised by the Engine Management System, so it must remain attached and connected electrically so that you don't get the EMS spanner light on the dashboard again. This took about half an hour all told and the car no longer stalls. I love the way this car drives but the engine is poorly engineered!