Do you actually mean concrete or is it block paving you have in mind. Concrete is hard wearing but inflexible. bear in mind that you will probably be covering you services , gas electicity and drains. Avoid cold callers and those with unmarked vans. Get recommendations from friends family and neighbours and have a look at previous work preferably something that has been down for a while as unevenness or poor settlement will show. Get free written estimates stating exactly what you are buying.