Genghis:
I have been known before now to use some incorrect and sarcastic RT
"G-AB, turning base and is now No.1, aircraft ahead has left the circuit".
It gets the message across, and doesn't endanger anybody
I sympathise, having also been there many times. But I would caution against the notion that other pilots on frequency (a) hear what you're saying and (b) have any idea what it means in respect of the location of your aircraft relative to theirs. In my experience the average PPL has very poor situational awareness. When this sort of thing occurs on an A/G frequency where the A/G operator is very passive (most of them in my experience), rather than be sarcastic, my policy is to address the other aircraft directly along the lines of "G-AB from G-CD, we are established on base leg and visual with you on a long final, what are your intentions?". Also contrary to CAP 413 I know, but I believe the key to all of this is to get people speaking to each other. I get completely fed up with the usual situation at A/G fields where you get:
"G-AB final 26"
"G-AB roger"
then a few seconds later
"G-CD final 26"
"G-CD roger"
without any traffic info given, let alone requests to the offending parties as to whether they can see each other.
Speak, or forever hold your peace!
NS