You need to find the underlying cause, you should not need to start changing things.
The Escos uses a two stage fan switch and is far superior to that of the terrible one fitted to the Sierra/Sapphire this is what causes problems with Sierras due to the massive span they have so if you fit one of these you will have more problems.
The MSD 'Uprated' fan switches are completely different to the standard ones and has a span of just a few degrees (Circa 92 degrees on and off at around 88 degrees) opposed to the 11 degree span of the original, it's the 82 degree thermostat you can buy not the switch.
The above about the fans applies to Sierra's though NOT Escort's.
You say it's fine when on the move, which leads me to ask if your fans are working, if not this will be causing the overheating when in traffic.
Where about's in London are you from?
Martin