If the garage needs more than 3 days to diagnose your car, then you get a hire car through RAC. Maybe if you book it in on a friday, its in for essentially 3 days??
That is what I done dropped the car off, phoned motability, they areanged a taxi to get me home and a hire car was delivered to my house next day. I think it was Enterprise, but not sure.
It worked for me, maybe others have done that as well, if you want to chip in?
I had the hire car for 2 weeks.