ebony,
At 4.2 I would have had 2 glucotabs and then eaten the crisps, don't forget that meters aren't 100% accurate, so you may well have been 4 or below by then. Also the fat content of the crisps would have had a slight delay in absorption, next time some fast-acting glucose then the snack!
Also, just remember as type 1's our insulin ratio's can change without explanation, so your ratio this week/month can be somewhat different the following, for now try reducing your tea time insulin by 2 units or increase your carbs by 2 cp.
Nigel