Is this - via A40/M40 - going to be a nightmare ?

Any idea on time ?

Leaving Harrow in the morning, returning evening.

The A312 is very slow going north from Northolt Target in the evening. Maybe
coming off at Polish War Memorial and going past South Ruislip station might
be better.

The OP might also try coming off at the Greenford B&Q exit, turning left
past Greenford tube station, past Westway Cross retail park, and then
getting onto Whitton Avenue and heading back towards Harrow that way.

It's a little detour, but it does avoid the queue getting off the A40 at
either Target or Polish - which can be as bad as 15-20 minutes or so.

Regards
Paul