Hey Dan,
All springs will feel stiff for a good few sessions, but then as you develop a good technique you will get more comfy using them. At 9 stones you are just over 57kg, we always recommend you go up 10kg, which means the right ones for you would be around the 67kg mark so the M70's would be the best set for you. If you went for the M60's you would still find them stiff for a couple of goes but then would be dissapointed once you got the hang of it. Stay with them and you will be glad you did!