Once I worked put how the shoes fit in the clips, they were great. Unfortunately, I was nearly at the top of the big hill until I figured that out

I really can be as thick as a brick sometimes!
I thought I was clipped in from the off. The connection was loose and wobbly, but as I've never worn clips before I didn't know what to expect. Anyway, on the hill my feet were wobbling around on the pedal, and I'm thinking "it felt better in my old trainers". Couldn't get any real power on. Got frustrated, stood up to try and stretch out the legs and 'click', the weight of me standing must have pushed the clips in properly. After that it was great.
Did some sprinting, and standing up on the hills and sprinting to get over some little humps and the clips definitely seemed to help.
Maybe next time I'll be clipped in properly from the off