One general tip is to choose an independent garage which is operated by the owner or their family. They have generally spent many years building the business and regard you as their personal customer. They will probably do a good job and be fair with you as they want you to go back and recommend them to others. The larger chains such as KwikFit don't usually care about customers so much as their priority is weekly turnover and not customer satisfaction.
My answer is a bit vague Muchloved but that's because the area is a minefield and we can only give you some very general tips. As Which magazine found in 2007, after 50 years of campaigning for good vehicle service it's still all down to pot luck. They recommended the independant mechanic or garage as giving the best chance of a good and fair job. There are no hard-and-fast rules and you can get caught anywhere, but if you find a good mechanic or garage then stick with them.
Also, what about asking around your neighbours. They usually know who's good and bad in the area.
Good luck.