I have 18 slots, 14 way, and when Solar panels fitted not enough room so second CU fitted. Seem to remember paying around £300 for parts, fitting was not paid for, done in a day, but ran out of time to fault find, I did that.
Only reason not done DIY was cost to notify, and a friend offered to do it. Back in 1980 my time charged out at £25 an hour, I did not get that, but can't see anyone charging less than £40 an hour, and can't really do two in a day, so at £600 one is getting a real bargain, and even double that is cheap.
But it does depend where, it was worth my while getting lodgings in Heathrow as wages were so much better than North Wales.