Can anyone please recommned someone to "top up" the air conditioning system on a car? It's a specialist job and ordinary garages don't do it.

Try Ichthus Motors, opposite Dalmain School in Brockley Rise.
They did a good job on my elderly Peugeot.
There's usually a fixed price for an aircon service that includes the fluid, so ask them. Aircon systems should be checked every 2 years or so and the service includes finding any leaks.
I believe some branches of Kwikfit have the necessary to do this.