Not used them before but drive past 4Front cars at Warninglid on the A23 most days. Seem to have some decent motors that are 7 sweaters in there
Before you start shopping for a 7 seater you may need to qualify how important is it that the second row middle seat is full size. If it is important your choice will be narrowed greatly.
The French have had this market sussed for years. You won't go far wrong with a Renault Grand Scenic or Citroën C4 Picasso. They seem to get the interior layout just right for big families. The Kia Sorento is another that won't break the bank new or used. Further up in price is the Ford Galaxy and at the top end the Audi Q7.
I would be looking at the first two to be honest. Tates in Portslade always seem to have Citroëns on offer.
We wanted 3 actual seats in the middle row and just about the only one we could find that did that was a Ford S Max. Had it for 2 years now, great car.