Welcome to the forum, Nigel. We need a few more facts to be able to discuss your readings:
(1) when was your 14.1 (I'm assuming mmol/l) reading? fasting? before / after breakfast?
(2) how many slices of bread in sandwich? how many ml of juice? then we can estimate carbohydrate content of meal in grams
(3) how long between eating and checking blood sugar level?
(4) what else have you been doing today?
(5) what insulin types and doses are you injecting? (I'm assuming you are using insulin, as you state you have type 1 diabetes)
First thought is that a rise of 3.4mmol/l might be normal for what you have eaten and that insulin dose might not be correct.