I'd certainly use a Letting Agent, at least to start with.
Rates vary, but for full management you could be looking at 15%. Plus there will be other fees - Agent's like the tenant to renew the agreement every 6 months and to charge the tenant and the landlord for this. Look in their terms and conditions as to how much this may cost. There's no actual need to renew, it shouldn't cost anything to let the agreement go Periodic if you are happy with the tenant, but Agents tend not to like this because they don't get paid. Also look for how much the inventory will cost - again, most Agent's will charge separately for this. Read their terms carefully.
Ask to see the credit checks and referencing on the prospective tenant - don't be fobbed off by the Agent with talk of the Data Protection Act, it's not relevant.
Most Agents wont get involved with anything more than writing to the tenant if they default on their rent. You will either have to know how to get your money through the Courts if necessary or go to a solicitor.
Many Agent's aren't actually very knowledgeable about Landlord/Tenant law. It may be best to go with one who's registered with an organisation such as ARLA.
Only sign a tenant up for 6 months initially, it's easier then to get them out if it all goes wrong. Don't be tempted by 12 month contracts, at least not initially. Likewise, be wary of tenants offering 6 months rent in advance.
Check out the Letting Agent as best you can. Times are difficult and some are going to the wall. How long have they been around? How many properties do they have rented out? Do you know any other landlord's locally who would recommend and Agent?
Make sure any deposit is protected - it's your responsibility not the Agent's. You can do it all online and it's free.
Allow for void periods in your calculations - this is the time the property is empty between tenants. Make sure you