I personally wouldn't ride fixed with a significant amount of weight, esp panniers. I'm not surprised you came off, it sounds like it was inevitable. And in the long run not good for your knees.
You could go singlespeed with a slightly relaxed geomentry (maybe a path frame?) which'll help you balance the weight.
But for town riding, fixed with a tight frame is best - you'll get the agility you need amongst the traffic.
Presumably you don't commute from the cotswolds, so have you thought about 2 bikes?
I personally wouldn't ride fixed with a significant amount of weight, esp panniers. I'm not surprised you came off, it sounds like it was inevitable. And in the long run not good for your knees.
You could go singlespeed with a slightly relaxed geomentry (maybe a path frame?) which'll help you balance the weight.
But for town riding, fixed with a tight frame is best - you'll get the agility you need amongst the traffic.
Presumably you don't commute from the cotswolds, so have you thought about 2 bikes?