Did a bit of research and found a nice little gadget....
I bought the Hella kit (Part no 8PE351230-841) of 4 for £18 because I didn't know which one to use but they are available individually for about £7, so ask for the 5/8 size one. Got it from Allparts.
Also found a much cheaper Sealey kit on the net for £8 +p&p part no VS0454.
Here's the one that did the trick:
Here are the pipes it disconnects:
Pipe #1, has a spring hidden in the ring to the left of the 2 green o-rings:
*** WARNING THE SYSTEM MUST BE EMPTY OF REFRIDGERANT GAS BEFORE DISCONNECTING, APPARENTLY THE PIPES SHOULDN'T DISCONNECT WHEN THE SYSTEM IS UNDER PRESSURE BUT DON'T TAKE ANY RISKS. THIS GAS CAN CAUSE FROSTBITE. IF YOUR SYSTEM HAS GAS THEN IT MUST BE DRAINED AND RETAINED BY A PROFESSIONAL BEFORE COMPONENTS CAN BE DISCONNECTED***
*** ALWAYS USE RUBBER GLOVES LINED WITH MATERIAL AND PROTECTIVE GOGGLES WHEN WORKING ON A/C WITH GAS STILL IN THE SYSTEM ***
1. Clip the gadget over the pipe join with the bit (protrusion) that shows thru the side slots pushing into the spring in the ring (it opens the ring up)
2. Pull the pipes apart, sorted!
If that didn't work then the spring is a bit stuck in place so:
3. Push the pipe #3 into pipe #1 while pushing in the gadget into ring of pipe #1.
4. Pull pipe #3 out.