I have a cooper s 2010 48k miles Only had it 2months . Driving in Glen Coe car started juddering and loss of power. Had to get recovery to come out and they said it was coil pack so got garage to replace and fit new plugs. Turns out I have a bigger issue 25psi in cylinder 4 rest 150. Psi. What would have caused this problem and how much am I looking at repair?

had call from garage they think it’s a burnt valve because end of spark plug has been burnt off.
I’m totally lost as what to do now

 

Most likely there was lots of carbon build up behind the valve and the excess heat retention caused your valve to burn up. Its not a fun job to R&R the valves because there is so much to remove and its very tight, I recommend putting the front end into "service mode", my granddaughter and I just did this job and the only reason it took a while is because of her schedule (high school student). I was amazed at how much junk was built up on the intakes.