Lawnmower Mecca fail!

Took my lawnmower in for service. It was running perfectly well but needed new blades so I thought I might as well get a service. Quoted me well over R3000 to replace rings and service. Clearly I declined this ridiculous quote as the mower was running fine. Asked them to just replace the oil and blades. On collection, oil was still black. Unfortunately I had already paid and the staff assured me oil had been changed. At home mower would not start and backfired. I bought new oil, new spark plug and changed them myself and cleaned the air filter. Voila! Mower runs perfectly well again. Of course the new oil is clear and obviously they did not even bother to change it despite having charged for it. Also, spark plug appears to have been sabotaged. Gap to small, gummed up and probably the wrong size (lawn trimmer smaller size but not sure if these run fine on a mower, but standard plug works perfectly well). I strongly suspect these guys are ripping off customers big time and see someone else had a similar experience. Please watch out for these guys or better yet steer clear.