Just received one of these (SIM Free, UK Version, Android 2.3 with Sense UI).
Can anybody point me towards the simplest and most easiest way to root this handset?
I would recommend the manual method. It's more complicated, but gives you a "better" root ( S-Off, SIM-unlock, SuperCID).
Just so that the above answer doesn't mislead any others. Please do not use the Visionary method. You may brick your phones.
If you are currently on Gingerbread then use this Guide by Setherio to downgrade to Froyo after which the phone can be rooted. You may also use the PDF available in the above link to do the same along with rooting your device.
If you already happen to be on Froyo with software version 1.3x then you can use the PDF to root your phone.