Colin, you were right !
Today I took my car to a small independent garage to get the air-con regassed.
They found a large leak. They fixed it. They found another leak(using just soapy water in a spray gun), they fixed that(right next to the exhaust manifold). They charged the system with new gas and Hey Presto, the Lexus is once again a beautiful place to be on a day like today.
The cost...............£40 all in. I gave them more.