Hi @Andy41, my way of reasoning out after breakfast BSLs (I am assuming the 18 mmol/l is at the 2 hour mark after eating breakfast) is that the level is a balance of how many carbs and types of carbs and protein in and when; how much insulin and how determined, what type/brand of insulin used and when; what factors are influencing insulin sensitivity; and other factors not under direct control such as any intercurrent illness, the season, weather etc.
This above and below is from my own personal exploration of things and experimentation. It was not written as professional advice or opinion. A dietitian, DSN/doctor are your advisors and the ones to provide answers.
1) Taking carbs as an example: If you look up mendosa.com - Glycaemic values (and definitions for GI and GL) - Cereals of GI > 55 and GL > 10, tend to raise BSLS more quickly than those with figures below those levels. And looking further, if you add say, milk to the mix, the GI levels are lower. So adding something with fat in it slows passage of food from stomach to small intestine (where absorption takes place). So one way I can influence and reduce myBSL rise after breakfast is to eat low GI/GL cereals in appropriate but not too large amount, with other additions like suggested in other answers like yoghurt, cheese, eggs etc.which may slow and flatten the BSL rise.
2) Also some of us have been advised/taught how to calculate units of short-acting insulin per number of carbs (in grams), so that if the breakfast intake varies, the insulin dose is altered to best "match' the amount of carbs eaten. There are courses run which teach persons this - others on site are more au fait with these UK run courses than me, an Aussie.
3) If you google 'graphs or pictures of insulin profiles', you can see how the amount of particular injected insulin in the blood rises over time. For one brand of short-acting (bolus) insulin this may show a peak at 1 to 2 hours after injection but with another, some 2 to 3 hours after injection. Of course these levels are the drug company's results of testing on people and so represent an average of what these insulins blood levels are. So .. if I have a bowl of high GI cereal in hot water plus a sweetened OJ for breakfast which gives me a peak BSL at say one and one-half hours and inject just before breakfast a particular bolus insulin which typically peaks in its effect at say 2 and one half hours, the BSL peak is not necessarily going to be matched and 'combatted' well by the insulin. And in fact if I, in frustration, keep upping the insulin dose more and more with the same breakfast I can end up with still a high BSL at one and one-half hours AND a hypo later as the insulin peak at 2 and one half hours pushed the blood sugar down. The hypo may cause a rebound of BSL as glucagon and adrenaline are released plus whatever I have eaten to combat the hypo and I go into high bsl range again!! So in the past I would talk with my doctor about this: 3 main options a) change to a quicker-acting insulin b) change to lower GI/GL food plus more fat to bring the BSL peak to nearer the insulin peak c) alter the timing of insulin injection vs intake of food (e.g. in the above example, have bolus injection, wait 45 minutes or so and then have breakfast so that the insulin peak best meets the BSL peak. Again, these are all things to discuss with your health professional team first.
4) There is a thing called the Dawn Phenomenon (DFP) and in the Home section of this site under 'Type 1' you can find some info about it. DP leads to a situation where our bodies may become more resistant to insulin (= lowered insulin sensitivity) in the early morning, from say 4 am approx. onward) and glucose may be released more readily from the liver. Not every T1D has this happen or not all the time according to the info. But a higher than normal BSL first thing in the morning (called fasting BSL) may be a sign of DP (among other causes). So if insulin works less well in the morning more insulin may be needed then for a given amount of carbs than say for lunchtime or evening meal. And if fasting BSL is high, that adds onto the subsequent rise of BSL after breakfast. The idea of some exercise on waking, as suggested above, is to try to improve insulin sensitivity. But if you are keen enough to test BSLs say at 4 am, 6 am and then just before breakfast, a couple of times, you then have some readings to discuss with your dsn/doctor and see what they suggest.
5) because of the BSL rises after food some TID, including on site, actually stick to a low carb diet (see on Home page under 'Food and Recipes'). Lower carb intake = less insulin required (as a general rule). Low carb diets are endorsed by the NHS, but usually I gather for T2Ds. However for some T1Ds low carb appears to work well.
6) the other easy to measure factor is weight. The more we eat the more insulin may be required (particularly if carbs are a major part of one's diet) and the higher one's weight goes. Added weight can affect insulin sensitivity, up goes the weight, up goes the insulin dose(a vicious cycle). Yes, if one exercises a lot you may well be able to keep weight in normal range but again another reason to check with your health team about how appropriate your diet is.
Best Wishes.
Thanks kitedoc. I’ve read this a couple of times now and take a few more bits of advice each time. I’m guessing you’ve got all this down to a fine art!
does it happen to you regardless? Before or after meals and or if you do fasted HIIT? 
