Car in for service - they ceased the engine...

Took my VW Polo blueline for a major service and timing chain on a Saturday morning expecting to have the car back by midday. Well I received a call from Leroy to say that the timing is out and they cant get compression -later that morning another call to say the motor is ceased and head needs to be skimmed!! Well...... how can that have happened when I drove the car in with no issues but for a service and timing chain replacement...... I had another mechanic I spoke to and was told that they must have put the timing chain on the wrong cog and therefore ceased the motor.!! Hence they had to send it to be machined and long story short i was given a "discount" for all the work they did - I should not have been charged for the ceasing of the motor as it was fine when I drove it in there..... Once i get the car back i notice the tappet cover is missing, i called and was told the car never had a cover when i brought it in!! It did, I know my car and open the bonnet regularly, would have known if there was no cover. Well he said she said so here I am without a cover and out of pocket to much more than was quoted..... I will never ever recommend them to anyone and if you do take your vehicle there, make sure you sit and watch them working on it !!!!