tempted to go via Pris to save time, should we?

[FredWorms]

Calais to Bordeaux?

Hmm

TBH, no

The problem is that if you go through at 2pm it might be quicker but go through at 4pm it will be slower. The danger is that at 2am there may well be roadworks going on.

DH recommends Rouen/Le Mans. I will check times and let you know

No.
Rouens, Le Mans definitely.

The default setting for my route planner is via Rouen and puts it at around 8 hours drive time.

Via Paris has roughly the same drive time but IMO & IME via Rouen carries far fewer risks of it going wrong. If you get a traffic jam via Rouen then you have a chance of getting round it by going off-piste. Via Paris then unless you know Paris like a Parisian taxi driver you are stuck with the main routes which will be full.

Plus the service stations round Paris are generally grotty!

Rouen, then we often stay at alencon on the municipal site as it is a pleasant town with a great pool. Wouldn't go via Paris, though I have directed all the way around the outskirts of Paris with a paper map and didn't go wrong - my drivers was very impressed