Thank you for this. It worked! I've tried various ways before, including an earlier version of SuperOneClick, but always failed.
A couple of things I did which may or may not have made a difference: after turning on USB debugging in the phone and connecting it to the PC, I didn't respond to the phone's prompts to choose Kies or USMT modes, just left it sitting there. After clicking the Root button, it all went fairly quickly until SuperOneClick said that Busybox was not found and did it want me to install it. I said Yes. After it had finished, I rebooted the phone. Titanium Backup now runs and successfully acquires superuser privilege.
Found UnlockRoot 2.3 which did the trick. You install it on your PC, run it, connect the GT-I5500 with the USB cable, select the "Kies" option when asked, and finally hit "Root" on the app window on the PC. I notice the PC app also has an "Unroot" button (which fact I found reassuring 8^).