Just to update this again, I have got another dash cluster with the correct mileage, and I have swapped the speedo and the odo/trip within over to my car. At first I thought everything was fine as the odometer seemed to be working fine, but I think it is now rolling over in kilometres, not miles. I know the distance of my journey and it is saying I have done more miles.
Furthermore, the speedo now does not show the correct speed, I don't think it's a case of the miles/kilometre conversion going wrong, as when the car is stationary the speedo shows 25mph. The faces are identical on the two clusters, and the part numbers are the same, so I don't know what is wrong now?
Also, the fuel gauge doesn't seem to be showing the correct amount, but I've got no idea how to prove if the fuel gauge is working.