Hi Wayne, welcome from the UK. I had problems with my speedo bouncing around after I had the dash board out, because I had not fully inserted the speedo cable fully home. I found it easier to remove the speedo cable from the gearbox end first, and this allows you to pull the dash board out alot further to gain access at the back. When you remove the cable out of the gearbox, just wrap some masking tap around the end of the drive gear so that the inner drive cable does not come out from the outer sheath. As the Sherriff said, you will have to push the cable in towards the back of the speedometer quite firmly and then press the two outer tabs in to release it from its housing. You may find tilting the cable from left to right as you pull it out.
I jacked the back of the car up and placed it on axle stands to remove the cable from the gearbox, and when you have finished the speedo cable job and placed the speedometer back on to its dash mounts with the 4 screws, go underneath the car and check that the inner drive cable has not pushed out from the outer sheath. Remove the masking tape, being careful not to pull the gear away from the outer sheath and rotate the gear with your hand and at the same time push the inner drive cable into the outer sheath. This will enable the square shaped end of the speedo drive to engage further into the speedometer and give you a positive drive connection. You can now check that the speedo works fine whilst the car is up on the axle stands, and that the needle does not bounce around when you put the car into drive. DO BE CAREFUL WHEN CARING OUT THIS LAST TEST, AND THAT NO ONE ELSE IS AROUND WHEN THOSE BACK TYRES START TURNING