I've had another go at the non functioning speedo on the XBR and I've now got it going!
As the CBR speedo drive wouldn't fit, and having read Hondapartsman's bit on the forum, I had a look at the tabbed washer in the wheel hub which turns the speedo drive. The tabs looked a bit flattened so I bent them up a bit with a scewdriver and now the speedo works fine
I don't know how long it's going to work for. The speedo drive still fells very notchy compared to the CBR one. I think what may of happened is that because the speedo drive is sticking slightly the tabs have gradually been bent back until they stopped engaging with the drive.

So I'm still looking to replace the drive ASAP, but at least it given me a bit of time to look for one.