Decided to return the Wellhouse faceplate, they refunded my money no problem and were very helpful.
Bought a faceplate off ebay, the-karaoke-queen, it arrived the next day and simply replaced the old km one. I spoke to the seller and he gave me a useful tip when fitting it.
Having got the speedo cluster out of the vehicle before removing the old faceplate gently lift the speedo needle over the needle stop and mark where it comes to a rest. Fit the mph faceplate and refit the speedo needle aligned to the mark then move it beyond the position of the needle stop and fit the needle stop. I did this and when I checked the indicated speed with my sat nav indicated speed it was spot on so I avoided having to faff around fitting the speedo needle while driving at 30mph
. David Elliot is a very helpful chap.