Depending on consumer unit being bought they do range in make model and quality.
But £70 should get you an okay dual RCD unit that will cover ten circuits.
Insist on dual RCD.
If there are no problems with your existing installation, it is conceivable that the change over can be completed in the day.
I generally do a EICR (electrical installation condition report) prior to change to make sure that the existing installation is suitable for the swap. In some cases repair or alterations are needed before the swap can be made or you could suffer power loss problems.
Your in a different part of the country to me and regional prices vary, but for a modern house with 7 circuits, with no historical electrical problems. I would be charging around the £400 mark.
That would include standard dual RCD unit, EICR and EIC (electrical installation certificate) and building control notification.
If any remedial work is needed, that would be quoted for on completion of EICR. Then addressed once confirmed you are willing to go ahead with the work. There would be charge for the EICR, if you decided not to go ahead with any remedial work and swap!
You may come across electrician that cannot self cert their work, (note:does not make them bad electricians) and they will have to have their work signed of by building controls, which requires an application and notification prior to work starting, which comes with quite a hefty fee.
So their price could be higher than that of an electrician that can self cert (note: does not make them a good electrician)
If you follow the link
http://www.competentperson.co.uk/
you can find self cert electricians in your area.
Hope this info helps!