However, I dealt with this one swiftly and efficiently by speaking Welsh to him. As we have caller display, the "International" gave him away and so I could start the conversation in Welsh right from the beginning. I wished him a "Bore da," (good morning) and he tried to launch into the usual spiel, but at the first pause I asked him politely (in Welsh) what he wanted. He ignored that and tried to continue for a little longer, but was clearly faltering by now, so when I explained, (still in Welsh) that I didn't want to speak English with him, he gave up.
So now I can tick "Say something in Welsh" off my daily to-dos. :)