Had a lesson again today and the advice on this thread really helped, especially in the canter which feels a lot more balanced and I managed to not lose any stirrups today
Still gripping a bit with my knees when I lose concentration for a minute but hopefully I should be able to sort that soon. Thanks for the advice
A very easy solution can sometimes be that in trot your rising higher than you need to, giving you the need to grip with you knee's. When you rise your hips should come beneath you in aline with the girth, which means you don't have o rise as much and you weight will naturally balance on the balls of your feet and allow the heels to drop. Use your girth as an alinement tool - a suggestion alot of grand prix riders have given me.