When you say 'rough idea' ... you really are holding your cards close.
Do you live in a bungalow or in a three storey town house ?
Is it easy access, or is a scaffold required.
Do you live in Knightsbridge or Selsey.
All of these factors will have an effect on pricing such a job.
For instance ... My friend is a builder and his day rate is £195 if it's local .. £225 outside the M25, and close to £400 inside the congestion zone. ( he charges by the day, nothing less for half a day ).
He says it's at least a days work for him and his labourer (another £150), and the materials should be around...
£250- 350 depending on the spec. of the materials .. subject to survey.
A neighbour recently had Anglian do Fascias, soffits and gutters and they charged £800 for approx 10 metres.
Or you could do it yourself for the cost of the materials.!