I'd use a light moisturiser in the daytime, firstly so it acts as a barrier between your skin and the make-up (which can help with acne as long as the moisturiser is a good one) and secondly to give the make-up a more even finish. Avene Skin Recovery Crea is best for problem skin, IMO - very gentle, non-greasy, and will help blemishes to heal. It costs £12.50 for 40ml from Boots and lasts a very long time (my current tube is nearly 5 months old and gets used 2-3 times per day, depending on whether I redo my make-up between uni and evening job). Their Extremely Gentle Cleanser is also great, especially for the morning. For oil control during the day, the new Kleenex blotting tissues are fantastic.
However, if you really want the acne to go away you need to get a GP appointment and push for a referral to a dermatologist. Your body has had enough time to adjust to the Mirena and the hormonal acne is still there, so your best bet is to see what topical treatments are available and maybe consider switching to a different contraceptive method if the acne still bothers you after other options have been explored. FWIW, while cystic acne is extremely hard to treat, most self-diagnosed "cystic acne" isn't really that - only a dermatologist or a GP with a special interest in acne will be able to advise you reliably on the best treatment.
If it turns out your acne isn't cystic, I can't recommend this thing highly enough: www.lumie.com/products/clear I'd tried everything my doctor was prepared to prescribe for my acne but this was the only thing that shifted it permanently. I can even wear make-up now without getting spots, whereas before I'd get lots of them starting up the day after taking it off. There are a few studies which support the technology's effectiveness, and my personal experience of going from moderate acne to none at all has been enough to convince me :).
There are also some small changes you can make to your life which could help tremendously. If you adjust your diet to include more oily fish and a lot less red meat and dairy then you may well see an improvement (see The Clear Skin Diet by Alan Logan for studies and explanations which back that up, it's actually pretty good). I cook vegetarian meals every other night and alternate between chicken and fish for my meat meals, which helps a lot (and is much cheaper). Adding spinach, pumpkin seeds, cannellini beans, sweet potato and lentils to your food and cooking with EVOO are all cheap and easy ways of making your diet kinder to your skin. There's a great book called The Clear Skin Cookbook (by a medicinal chef, can't remember his name) which is a good starting point for changing your diet. I also swapped tea with milk for loose leaf green tea and white tea, and feel much better for it.
Sorry, this turned into a very long post - as you can probably tell, I've spent such a long time getting my skin sorted that I've turned a bit evangelical :).