Hi Kevvy,
In November 2007, I was then a landlord and used the same Corgi Engineer every year for boiler servicing and a safety certificate.
At that time, he did a power flush, replaced a pump in the boiler and put in an outside drain valve.
The price came to £350 labour plus the cost of the pump, for the whole job. Shouldn't be much more this year.
Motorsed rad valves are available from £30 to £50, and the standard valves for about £6 to £8 a pair at Plumbers merchants. No need to pay any more than that for parts. You can buy at the trade counters yourself, as a pensioner you probably don't have money to waste on BG's or other firms profits. Good Luck