Car Repair - not fit for purpose - faulty part

[Blue123]

my car was taken to a garage, it needed a new turbo - they told me they only fit genuine ones hence the price, we had to wait some time for it to arrive from Germany they said - the work was done they told me it was not guaranteed? they would not let me have the old turbo back and said i would ahve to pay a surcharge if we did for it. they kept referring to me not having a service lately ( I had but the book simply had not been stamped which they found ) but regardless - I took the car away and it was clear it was still not working ( warning light was on) In hindsight It is my belief they knew it was not working right and that is why they wrote ''work not guaranteed'' on the invoice - I kept the speed down to 30mph and avoided motorways of course. I was commuting to work - I contacted garage and they said they were too busy to look at it and the mechanic was away and fully booked afterwards - so I was advised by another mechanic to get the computer reset which i did and the warning light went away. I decided to sell the car , and the buyer contacted me the next day to say there was a fault when he drove it fast on the m/w - it was a long distance away so he took it to a garage and they did various tests and then said it needed a new turbo. The new owner let me know the old turbo ( that was removed was not a genuine one and had not been fit properly/faulty) they supplied me with pictures. I paid for this work to be done as it was the right thing to do, in fact it was cheaper than the one I had paid for ( paid on a card) and that was not a genuine one - the car has been fine since and I was thanked.
I sent a letter to the garage (14 days) saying they had not fit a genuine part and not properly ( despite me paying for it) and i have since paid for the work to be done properly again with a genuine part! they phoned me and said I shouldve gone back to them ( I did contact them but they didnt want to do teh work) they said they woudlve arranged the work else where with one of their contacts where I had sold the car to. I was then sent a letter by a solicitor firm that they are covered with stating I shouldve given the garage the chance to rectify and didnt get the car serviced etc etc - The issue I have is I then went into hospital and have been too unwell to deal with this - is there a time scale I shouldve responded to their solicitor or can I simply take it to court small claims as the fact is due to poor workmanship and fitting a non genuine part I had to pay again to get the work done properly.