Used Car Issue - would like to confirm my rights

Hello everybody

Wasn't sure if this should have gone in motoring or consumer rights...apologies if it's in the wrong bit!

I purchased a BMW 1 Series 118i three weeks ago, from a registered dealer. I paid more for the car because I wanted something with low mileage, which would be more comfortable and hopefully a little bit better on fuel than my previous Mini Cooper.

Unfortunately, I am getting well below the average mpg for the car, which is currently sitting between 20 and 21 mpg. I drive the car very sensibly, the tyres are all where they should be and the car (allegedly) received a full service prior to me collecting it. I drive a mixture of motorway and urban driving and the absolutely best I have seen it at is 23mpg. I have filled the car twice (so I could be sure it wasn't just me and get a reasonable idea of mpg without relying on the trip computer) and it has remained the same.

I contacted the owner who asked me to take the car in for him to have a look at. While with me, he called a garage who said that the mpg was perfectly reasonable. I have checked forums, spoken to friends with the same car, even called BMW who have all said it doesn't sound right.

On owning the car a few days, a yellow stain also appeared on the back wheel arch. When I mentioned this to him, he tried to imply somebody had knocked my car. This is clearly not the case and more likely an attempt at them polishing up an existing knock, badly.

Another issue (although this is purely on a hearsay basis, so almost irrelevant!) is that the car was sold to me with two former owners, when in fact there were three. The seller lied to me about this.

I explained that I did not feel 20mpg was acceptable for a car that age, with so few miles, which has been recently serviced. He said he would speak to his manager and call me back.

He didn't. I called him and he said he would call me later as was with a customer. Again, he didn't. I sent a very reasonable email requesting that he calls me but still, I have heard nothing.

I feel a little bit upset. I spent a lot of money on this car and went to a dealer as I thought it would give me better quality and protection.

I have four days left before I owned the car for 30 days. I would like some advice as to whether or not I could refuse the car under the Consumer Rights Act, based on the fact that the car is not running as it should? Or is MPG something that has such a massive spectrum anyway that it isn't good enough grounds?

I paid a deposit on my credit card, then the rest via a debit card.

Thank you in advance for your responses.