I recently had a job done on my central heating. A Worcester Bosch 24Ri condensing gas boiler, 24kW output should come in at about 1200; you can add on at least the same amount again for a circulating pump, pipework, corrosion inhibitor, and labour. You should insure the fitting includes gas flue analysis. Thermostatic valves are about 35 each. Power flushing should cost about 175. Then add on VAT at 13.5%. Don't forget to claim for an income tax credit for qualifying expenditure under the HRI scheme.