Update on this, and thanks to posters for the reponses!
I've used a label maker I had to make some little '10' '20' etc. numbers and have placed them on the current KM/H dial. This is all that is needed for MOT - Northern Ireland appear to be the only UK/Ireland MOT place enforcing this rule, but some European directive states that speedos must be dual display - especially import in NI I suppose because we have a road border with a EU KM/H country!
If I can get a Lexus IS200 cluster I'll try doing a straight swap and see how it goes - concerned that the mileage will be way off though.
My other option is to install a speedo converter / speed limit defender - it will make the speedo dial read in MPH, but the base line odometer (with 140,000km) will then start to read miles. So I'll have to have the awkward conversation when I sell the car 'it's not 150,000 miles, it's 140,000 kilometers plus 10,000 miles) !