We normally work on a daily or hourly rate but will consider fixed price with a well defined requirement specification when there's a budget.
We supply progress reports and can invoice weekly, or can work on a 30-40-30 plan (30% payment at start, 40% midway normally showing some partial deliverable, and remaining 30% at delivery).
Note: When comparing us to cheaper providers, please keep in mind we are not a outsourcing shop contracting cheap partly-skilled labourers in poor countries. Neither do we employ any learners to reduce our overheads. We develop everything in UK using certified and well experienced seasoned developers, and take extra effort to properly structure and document code with good comments to assist future enhancements.