I can answer my own question.....!!! GR Vodafone, COSMOTE, and TIM(that was the one I forgotten)....all have the same roaming rates!!(what a surprise). The better news is that whilst it cost £1 a min to make a call and 60p to receive one.....the rates seem to have changed to 70p to dial out and 30p to receive. Still high, but thanks to the EU, not as big a rip-off as it was.
I got this info from the Orange website...and the GSM Europe website has prices across Europe, including Corfu and Greece(though I don"t think they"ve updated the Greek tariffs).