Thanks for your reply.
The garage is no longer interested to look at this vehicle as they believe that the work they've done (skimming the head) is proper and there is no problem with it and anything else is not their problem. They confirmed that they will guarantee and honour their warrantee for that work only if there is any problem with it. Their argument now is it must be that the block is twisted and that's what's causing the problem?!
The vehicle has done less than 200miles since it came out from the repair.