If you are looking to outsource your marketing to a third party then you need to ensure that you find a company or an individual that you can trust and reply upon. Often the best way to do this is to go off recommendations, but this may not always be possible.
You may find that there is quite a difference in the prices charged by marketing agencies, individuals and agencies abroad. You may be approached by several companies that are based in places such as India that are willing to do your marketing for a fraction of the cost that you would pay in the UK, but you do need to be careful as you may find that due to the time difference there is quite a delay in being able to get in touch with them or getting work to and from them.
The best way to find good marketing people is to see their results. You should never feel bad to ask to see what work they have done previously and what results they have achieved. You can sometimes also ask for if you are able to speak to another one of their customers to get feedback as to what they are like to work with.