While they can probably help advice what service level is right for you, you should consider whether you want them to do specific tasks or take care of all your finances. If you do not wish (or are unable) to do the following, then you should investigate an accountant that can fulfil these duties.
Tax. Are you happy to manage your tax returns? Do you know how to file them accurately and on time, and ensure timely payment is made to the government?
Accounts. Do you want to create your management and financial accounts? Do you know the legal requirements, and are you able to file them on time?
Paperwork. Do you like keeping on top of your invoicing, receipts etc.? If you are not a paperwork person, then you should probably pass this over to your accountant.
Advice. Do you want someone who understands your business and your industry – someone who can advise you when you discuss your business growth ideas?