Had a shop around and managed to find a direct replacement Denso sensor for £38 delivered from DIYcarserviceparts. Cracking price compared to the dealer price.

I was a bit concerned about whether it was genuine, however, it turns out the company is part of The Green Spark Plug Company, who I've used for a number of years and always had great dealings.

Incidentally, whilst cross referencing part nrs etc it appeared on the Autodoc website that the upstream and downstream sensors are the same part nr ? Not sure that's correct, but did order the correct downstream one for my car.

Great info