It doesn't sound too expensive, just make sure the electrician is on here: http://www.competentperson.co.uk/ or alternatively on the schemes website which they say they are registered with.
Oh,
And can you ask them to find out what that green and yellow wire is doing on the bottom of the existing RCD?
Oh,
And can you ask them to find out what that green and yellow wire is doing on the bottom of the existing RCD?