 Pressure derived fractional flow reserve FFR is recognised as being the gold standard method of assessing the physiological significance of angiographically intermediate lesions. A grey-zone exists between the originally validated cut-off for ischemia of 0.75 and the conventionally adopted cut- off of 0.80. Pilot data from our centre has suggested that only 1 in 3 coronary arteries with grey-zone FFR values demonstrate myocardial perfusion defects on stress cardiac MRI and others have suggested that the clinical outcomes in patients with grey-zone FFR are favorable with medical therapy alone. As such, stenting all lesions with grey-zone FFR as currently recommended may represent over-treatment and could attenuate the overall benefit of an FFR strategy. In addition to this there are flow derived resistance indices of stenosis severity that have superior diagnostic accuracy and may be helpful in correctly classifying patients with grey-zone FFR. In this study we will a comprehensive analysis of lesions with grey-zone FFR values 0.75-0.82 inclusive using invasive hyperemic pressure, flow and resistance derived indices of severity with quantitative and qualitative 3T perfusion MRI to enable identification of the best invasive predictors of true perfusion defects on 3T cardiac MRI. Patients will be randomised to optimal medical therapy alone versus optimal medical therapy with PCI and followed up for the primary endpoint of anginal control as measured by the Seattle Angina Questionnaire at 3 months. 