Hi. I actually work on the show... so here's the truth!!
The work IS done in 60 minutes... absolutely no cheats allowed (more's the pity).
We are allowed to 'prep' during the morning of the filming, ie clear the rooms out, measure for shelves and where holes need to be drilled, cut the carpets to size etc, but that's it.
We start filming at 2 in the afternoon and have a half hour tea-break after the first 30 minutes. if we've been clever and all the painting has been finished then it tends to dry during the tea-break. If the weather is awful then the tea-break tends to last a bit longer to give everyone a chance to prepare for the second half and the paint to dry a bit.
The last 30 minutes are as manic as they look and we have to have everything cleared away from the gardens and the house hoovered before the surprisee arrives home at 5ish.
Hope that helps!!
(and anyone wanting a makeover should phone the number on the ITV website... the production team really struggle to find suitable properties and everyone has a chance)