Too far away to help but in commissioning the work ensure that the electrician is a member of one of the part P bodies – ie NAPIT, NICEIC, ELECSA to name a few. You should expect an Electrical Installation Certificate to be completed and supplied to you and the work notified to building control on your behalf, which you’ll have a copy of.
A CU change should take around a day assuming a normal 3 bed type house and the proper testing is undertaken (often longer than the CU change itself). The CU should, in most circumstances, have 2 RCDs covering all circuits.
The cost of a fully populated 10 way CU is around £100 plus vat.
Any questions/concerns, give me a shout.
Rich.