it also depends on how much of the work you are prepared to do yourself. if you expect them to look after every bit of trivial interaction with revenue then it will cost you more. i prefer to do the simple things myself (vat for example) and keep careful records and organise my paperwork properly so that when it comes to doing the accounts and revenue forms, it only takes the accountant a few hours at the most. they should charge you less in this case.
Most accountants would prefer this, as they do not have staff to perfrom this work
Accountant's offices are usually combined of qualified and unqualified staff
The qualified staff usually have a broadbase of experience and their charge out rates reflect this accordingly and training staff are not usually that far behind, or are their charge out rates
Any good practice I have worked for had an external bookkeeper who they referred work to
This keeps costs for the client down and profitability of the practice up