Thanks Guys! Check my tyre pressure today-----only 32psi all round! No wonder I only manage less than 300miles with a full tank of petrol! Strange that I didn't spotted that. I had just been to the petrol station 2 days ago.
Instead of going to the gym today, I pump-up all four tyres to 40psi with my footpump and give the car another good clean. Not a bad way to lose some calories!
Took the car out for a quick spin, the ride had not deteriorated despite the extra 8psi on each tyres. The ACC must be working very hard!