Poor service

we took our X5 in to have the battery tested. they checked the charging and found it to be 14v. after putting a slave battery on the front and removing to old battery they installed the new battery. the alternator light was then on and overcharging at 18v. i spoke to various auto electricians and was told that the slave battery was possibly connected wrong and blue the rectifier. Llewelly refuses to except a possible mistake from his staff. i now sit with a hefty bill to prove this with no cooperation from battery centre