Graham_07
A lot of tax advisors and accountants charge by hourly rates. However, some, like myself quote a fixed fee in advance for specific work where the work can be quantified.
I would quote a fixed fee for most compliance work - tax return completion, accounts preparation etc.
Where a lot of unquantified research was involved, I would quote an hourly rate, but indicate how many hours I thought it would take.
PM me if you want more detail.