Problem with a used car

The Background
I traded in my old car for a Y Reg BMW at a local dealer on the 17th January. The price of my old car covered the cost of the BMW, plus some extra. I received a cheque from the dealer for the difference, that cheque has now been cashed and cleared.

The Problems
The very same day i picked the car up, I parked it on a hill and realised the handbrake did not work. I returned the car the garage and they assured me it would be fixed.

The following day I left for a trip the US for work, knowing I would be gone for 10 weeks. I gave the garage the contact information for my sister as she would be using the car in my absence.

During the time I was away, my sister receievd the car back, and returned it to the garage numerous times because of numerous problems.

The first time, the following was fixed/replaced;

TIMING CHAIN + GUIDES + TENSIONER
COIL PACKS
SPARK PLUGS
WIRING CHECKS
OIL & FILTER
CYLINDER HEAD GASKET
CYLENDER HEAD OVERHAUL
NEW HEAD BOLTS

During the time I was away, the car was in the garage for between 7 & weeks out of those 10. There was no offer of a hire car, and talk of a refund was laughed off on more than on occasion. While I was away I called the garage myself and tried to speak to the salesman, he was dismissive & rude and sarcastically told me "he looked forward to my letter" (regarding the return of the car) before hanging up the phone.

Only 2 weeks ago my sister put in writing to them that she gave them 7 days to have the car in full working order, or we would expect a full refund. 8 days later she got the car back. She only drove it home from the garage, and did not use it at all. She was told by the mechanic doing the work that "the best thing he can do is get rid of this car".

I came home yesterday, and drove the car. It runs horribly, and idles only just above 500RPM. It will occasionaly skip between 500-1200RPM rapidly when stationary. If the drop from ~1200-500 happens quickly enough, the car can completely cut out, even when the clutch is flat on the floor.

I called the garage today and asked to speak to the manager. I was told he was with a customer and would call me right back. I waited 3 hours before driving the car to garage to speak to him (on the way, the car cut out 3 times while slowing down for lights).

When I got there I was told the manager was on holiday and hadn't been in all day. I was also told I should be dealing with the salesman who sold me the car (who was also not in at that time).

I left my contact details and was promised a call would happen tomorrow morning.

Fast forward to only 30 mins ago when I was driving the car home (as much as the car is horrible to drive, I still need to get around), the inside of the car filled with smoke, and smoke bellowed from the engine. I immediately pulled over and shut it off. After it cooled, I turned it back over and it seemed ok. I immediately drove it to the garage and parked it on their forecourt with the intention of returning in the morning to ask for my money to be refunded.

The car has a 3 month warranty, which runs out on 17th April. I leave for work again on the 8th April, so I only have 5 days to fix this situation.

What are my rights here? I really don't want this car anymore, as I can't see there being anything but problems with it. I would like a full refund.

I've been told that because each time the car has been returned to them, and they are attempting to address the problems, that there isn't a trading standards issue. Does the following not apply

(2)If the buyer requires the seller to repair or replace the goods, the seller must—
(a)repair or, as the case may be, replace the goods within a reasonable time but without causing significant inconvenience to the buyer;

Any help you could give me would be appreciated. I plan on taking a letter of refusal for the car to the garage in the morning.

Thanks

P.S. I should maybe add, the garage in question is the one related to this article ... dailyrecord.co.uk/life/consumer/consumer-problems/2011/12/04/luxury-car-fraudster-returns-to-selling-motors-after-conviction-86908-23609575/