Hi Emrys, I personally am a great fan of Miracle Grow, I find very it a very good feed, especially for its folia feed properties providing that you apply it in the evening and not with the sun out (when you would scorch the leaves)
I grow the full range of shrubs and find the results since switching several years ago quite good and find that I can control the feeding better ie, down here in Cornwall with the very strong coastal wind we get, we need our plants to go into the winter quite tough to minimise dammage, so I feed up until the end of July and then let the plants toughen up a bit for the rest of the summer before the winter and I found that if I used a long term slow release fertilizer incorporated in the compost that I lost that control. I too top dress with growmore in the spring but remember that it does have added lime so don't use it on cacifuge plants such as Pieris, etc. I would suggest in gravel that you don't broadcast the growmore but scatter about one teaspoon full around each plant in April. You don't say where you live, as this will have quite an effect on your feeding strategy?
Best wishes lannerman