I have a 2005 MG ZT-T diesel with 69,000 miles - up to the last 3000 miles the needle on the engine temp has always stayed at the half way bar on the gauge no matter what kind of driving. However, at motorway speeds it now dips to the next bar down but slowly creeps back to the halfway point and never higher when in slow traffic - Fuel consumption remains mid to high 40s and no leeks detected - Should I be concerned ?!