2005 Vauxhall Astra incresed oil levels

took my car to have a couple of bits done at the garage ,he advised me that he had to take about a litre of oil out as it was to high, and told me that it could have diesal leaking into it, have checked now for the last three weeks seams ok but i think its creaping up again, was told may have leaking injectors , and would cost around 1200. 00 to fix ,i dont want to spend that much rather put it towards a new one, the car seams to be running really well better than before ,dont seam to be loosing fuel not that ive noticed ,iam getting so stressed out about it i do quite a long journey to work about 350 miles a week, would it be ok to change the oil often or what i dont know what to do with it got no smoke comming out no noises any ideas , only got about 5 months before i get rid .